Fix CLI formatting for empty Blueprint error messages (#3615)

## What?

Preserves diagnostic details when the CLI catches an `Error` with an
empty top-level `message` but a populated `cause` chain.

This fixes `wp-playground-cli run-blueprint` printing only:

```text
Error:
```

for Blueprint `runPHP` fatals whose PHP failure details are available
through the nested cause.

Fixes #3614.

## Why?

The CLI `describeError()` helper returned `error.message` immediately
for `Error` instances. When that message was empty, it never inspected
`cause`, even though the Blueprint/PHP failure details were present
there.

## Evidence

A Homeboy rig was added first to reproduce and measure this path:

- Repro harness: chubes4/homeboy-rigs#107
- Baseline run: `79d1d8c6-0bb5-4abb-8e18-5467e708cff2`
- Candidate run: `52652822-759d-438c-a56e-8d4b6605dc33`

Baseline metrics:

```text
control_exit_code=0
fatal_exit_code=1
fatal_blank_error=1
fatal_has_php_diagnostic=0
```

Candidate metrics:

```text
control_exit_code=0
fatal_exit_code=1
fatal_blank_error=0
fatal_has_php_diagnostic=1
```

Candidate output includes the PHP fatal:

```text
Error: Error — caused by: Error when executing the blueprint step #1: PHP.run() failed with exit code 255.

=== Stdout ===
Fatal error: Uncaught Error: Call to undefined function playground_cli_missing_probe_function()
```

## Testing Instructions

```sh
npx nx test-playground-cli playground-cli --testNamePattern="describeError"
```

Also verified with the Homeboy rig:

```sh
homeboy bench --force-hot \
  --rig playground-cli-diagnostics \
  --path /Users/chubes/Developer/wordpress-playground@improve-runphp-error-output \
  --scenario playground-cli-runphp-errors \
  --iterations 1 \
  --shared-state /var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/playground-cli-final-confirm \
  --output /var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/playground-cli-final-confirm/bench-output.json
```

## AI assistance
- **AI assistance:** Yes
- **Tool(s):** OpenCode (GPT-5.5)
- **Used for:** Diagnosed the CLI formatter issue using the Homeboy
repro rig, implemented the minimal formatter fix, added focused test
coverage, and ran verification.

Author: chubes4

packages/php-wasm/universal/src/lib/error-reporting.ts

@@ -47,6 +47,98 @@ export async function prettyPrintFullStackTrace(e: any) {
 	process.stderr.write('\n');
 }
 
+/**
+ * Describe an error for display. Handles Error instances, Comlink-serialized
+ * plain objects (which lose their Error prototype during worker thread
+ * transfer), and arbitrary values.
+ */
+export function describeError(
+	error: unknown,
+	seen = new WeakSet<object>(),
+	depth = 0,
+	options: { suppressGenericErrorName?: boolean } = {}
+): string {
+	if (depth > 10) {
+		return '[Max error cause depth exceeded]';
+	}
+	if (error instanceof Error) {
+		if (error.message) {
+			return error.message;
+		}
+		return describeErrorObject(error, seen, depth, {
+			...options,
+			suppressGenericErrorName: true,
+		});
+	}
+	if (error && typeof error === 'object') {
+		return describeErrorObject(error, seen, depth, options);
+	}
+	return String(error);
+}
+
+type ErrorLikeObject = object & {
+	name?: unknown;
+	message?: unknown;
+	cause?: unknown;
+	errno?: unknown;
+	code?: unknown;
+	stack?: unknown;
+};
+
+function describeErrorObject(
+	error: ErrorLikeObject,
+	seen: WeakSet<object>,
+	depth: number,
+	options: { suppressGenericErrorName?: boolean } = {}
+): string {
+	if (seen.has(error)) {
+		return '[Circular error cause]';
+	}
+	seen.add(error);
+
+	// Comlink-serialized errors arrive as plain objects like
+	// { name: 'ErrnoError', errno: 20 } with no .message.
+	const parts = [];
+	if (
+		error['name'] &&
+		!(options.suppressGenericErrorName && error['name'] === 'Error')
+	) {
+		parts.push(String(error['name']));
+	}
+	if (error['message']) {
+		parts.push(String(error['message']));
+	}
+	if (error['errno'] !== undefined) {
+		parts.push(`errno: ${error['errno']}`);
+	}
+	if (error['code'] !== undefined) {
+		parts.push(`code: ${error['code']}`);
+	}
+	if (error['cause']) {
+		parts.push(
+			`caused by: ${describeError(
+				error['cause'],
+				seen,
+				depth + 1,
+				options
+			)}`
+		);
+	}
+	if (parts.length > 0) {
+		return parts.join(' — ');
+	}
+	if (typeof error['stack'] === 'string') {
+		return error['stack'];
+	}
+	// Last resort: JSON-serialize the object so we at least see
+	// what fields it has.
+	try {
+		return JSON.stringify(error);
+	} catch {
+		return String(error);
+	}
+}
+
 export function printResponseDebugDetails(response: PHPResponse) {
 	// Print a short summary of what we have:
 	process.stderr.write(

packages/php-wasm/universal/src/lib/index.ts

@@ -18,6 +18,7 @@ export {
 	printDebugDetails,
 	prettyPrintFullStackTrace,
 	printResponseDebugDetails,
+	describeError,
 } from './error-reporting';
 export { UnhandledRejectionsTarget } from './wasm-error-reporting';
 export { HttpCookieStore } from './http-cookie-store';

packages/php-wasm/universal/src/test/error-reporting.spec.ts

@@ -0,0 +1,49 @@
+import { describeError } from '../lib/error-reporting';
+
+describe('describeError', () => {
+	it('falls back to Error cause when message is empty', () => {
+		const error = new Error('', {
+			cause: new Error(
+				'Error when executing the blueprint step #1: PHP.run() failed with exit code 255.'
+			),
+		});
+
+		const description = describeError(error);
+		expect(description).toContain(
+			'Error when executing the blueprint step #1'
+		);
+		expect(description.startsWith('Error —')).toBe(false);
+	});
+
+	it('terminates circular cause chains', () => {
+		const error = new Error('');
+		error.cause = error;
+
+		expect(describeError(error)).toContain('[Circular error cause]');
+	});
+
+	it('preserves empty-message formatting through nested causes', () => {
+		const error = new Error('', {
+			cause: new Error('', {
+				cause: new Error('Inner failure'),
+			}),
+		});
+
+		const description = describeError(error);
+		expect(description).toContain('Inner failure');
+		expect(description).not.toContain('Error — caused by:');
+	});
+
+	it('describes local fields before cause', () => {
+		const description = describeError({
+			name: 'ErrnoError',
+			errno: 20,
+			code: 'ENOTDIR',
+			cause: new Error('Inner failure'),
+		});
+
+		expect(description).toBe(
+			'ErrnoError — errno: 20 — code: ENOTDIR — caused by: Inner failure'
+		);
+	});
+});

packages/playground/cli/src/run-cli.ts

@@ -15,6 +15,7 @@ import {
 	exposeAPI,
 	exposeSyncAPI,
 	printDebugDetails,
+	describeError,
 } from '@php-wasm/universal';
 import type {
 	BlueprintBundle,
@@ -791,28 +792,7 @@ export async function parseOptionsAndRunCLI(argsToParse: string[]) {
 			if (debug) {
 				printDebugDetails(e);
 			} else {
-				const messageChain: string[] = [];
-				const seenErrors = new Set<Error>();
-				let currentError: Error | undefined = e;
-				for (let depth = 0; currentError && depth < 20; depth++) {
-					if (seenErrors.has(currentError)) {
-						messageChain.push('[Circular error cause]');
-						currentError = undefined;
-						break;
-					}
-					seenErrors.add(currentError);
-					messageChain.push(describeError(currentError));
-					currentError =
-						currentError.cause instanceof Error
-							? currentError.cause
-							: undefined;
-				}
-				if (currentError) {
-					messageChain.push('[Error cause chain truncated]');
-				}
-				console.error(
-					'\x1b[1m' + messageChain.join(' caused by: ') + '\x1b[0m'
-				);
+				console.error('\x1b[1m' + describeError(e) + '\x1b[0m');
 			}
 		} else {
 			console.error('\x1b[1m' + describeError(e) + '\x1b[0m');
@@ -821,60 +801,6 @@ export async function parseOptionsAndRunCLI(argsToParse: string[]) {
 	}
 }
 
-/**
- * Describe an error for display. Handles Error instances, Comlink-serialized
- * plain objects (which lose their Error prototype during worker thread
- * transfer), and arbitrary values.
- */
-function describeError(error: unknown): string {
-	if (error instanceof Error) {
-		if (error.message) {
-			return error.message;
-		}
-		const parts: string[] = [];
-		if (error.name && error.name !== 'Error') {
-			parts.push(error.name);
-		}
-		const obj = error as Error & Record<string, unknown>;
-		if (obj['errno'] !== undefined) {
-			parts.push(`errno: ${obj['errno']}`);
-		}
-		if (obj['code'] !== undefined) {
-			parts.push(`code: ${obj['code']}`);
-		}
-		if (parts.length > 0) {
-			return parts.join(' — ');
-		}
-		return error.stack || String(error);
-	}
-	if (error && typeof error === 'object') {
-		// Comlink-serialized errors arrive as plain objects like
-		// { name: 'ErrnoError', errno: 20 } with no .message.
-		const parts = [];
-		const obj = error as Record<string, unknown>;
-		if (obj['name']) {
-			parts.push(String(obj['name']));
-		}
-		if (obj['message']) {
-			parts.push(String(obj['message']));
-		}
-		if (obj['errno'] !== undefined) {
-			parts.push(`errno: ${obj['errno']}`);
-		}
-		if (parts.length > 0) {
-			return parts.join(' — ');
-		}
-		// Last resort: JSON-serialize the object so we at least see
-		// what fields it has.
-		try {
-			return JSON.stringify(error);
-		} catch {
-			return String(error);
-		}
-	}
-	return String(error);
-}
-
 function getMountForVfsPath(
 	mounts: Mount[],
 	vfsPath: string