dispatch: propagate field-name/count mismatch as RpcError

Conform-failure was being silently swallowed for both transports after
61025e1/589a95f, which let exchange handlers run on a wrong-shape input
batch. For column-name/count mismatches the handler then either picked
up the wrong column by index or hung the client (the conformance test
TestExchangeCastCompatible::test_cast_incompatible_column_name[pipe]
deadlocked end-to-end and tripped the 90s CI step timeout).

Tighten the catch in both src/dispatch/stream.ts and
src/http/dispatch.ts: TypeError (the only failure mode for field
name/count mismatch in conformBatchToSchema) is rethrown so callers see
a structured RpcError. Other conform failures (numeric cast issues for
dynamic-input handlers) still fall through with the original batch;
handlers that genuinely bind their input shape per call should set
state.__inputSchema so the conform check is bypassed entirely.

Local: 127/127 unit + 898/898 conformance pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rustyconover

src/dispatch/stream.ts

@@ -138,6 +138,13 @@ export async function dispatchStream(
         try {
           inputBatch = conformBatchToSchema(inputBatch, expectedInputSchema);
         } catch (e) {
+          // Field name/count mismatch (TypeError) is a hard contract violation —
+          // propagate as an RpcError so callers see a structured failure instead
+          // of a downstream hang or silent garbage. Other conform failures (e.g.
+          // type-cast issues for dynamic-input handlers) fall through with the
+          // original batch — handlers that bind their input shape per-call
+          // should set state.__inputSchema so the conform doesn't run at all.
+          if (e instanceof TypeError) throw e;
           console.debug?.(`Schema conformance skipped: ${e instanceof Error ? e.message : e}`);
         }
       }

src/http/dispatch.ts

@@ -277,6 +277,10 @@ export async function httpDispatchStreamExchange(
       try {
         conformedBatch = conformBatchToSchema(reqBatch, inputSchema);
       } catch (e) {
+        // Field name/count mismatch is a hard contract violation — surface it
+        // as an error rather than letting handlers see a wrong-shape batch
+        // (mirrors the subprocess dispatch in src/dispatch/stream.ts).
+        if (e instanceof TypeError) throw e;
         console.debug?.(`Schema conformance skipped: ${e instanceof Error ? e.message : e}`);
       }
     }