feat(core): extend error metadata handling in with() method

Refactored `DefinedError.with()` to improve metadata merging logic and type safety. Updated tests to verify region-based metadata inclusion.

Author: tarnishablec

packages/core/__tests__/caller-trace.test.ts

@@ -89,7 +89,7 @@ describe("ThatError Location Anchoring", () => {
                  * 💡 THE ANCHOR POINT
                  * We call .at() here to explicitly mark this line as the "Crime Scene".
                  */
-                return AppError.SYNC_ERR().with(void 0, {stage: 'init'});
+                return AppError.SYNC_ERR().with(void 0, {stage: 'init'}).with(void 0, {region: "CN"});
             }
 
             const err = businessFunction();
@@ -100,7 +100,7 @@ describe("ThatError Location Anchoring", () => {
             expect(topFrame).toContain("businessFunction");
             expect(topFrame).toContain(currentFileName);
 
-            expect(metaOf(err)).toEqual({stage: 'init'});
+            expect(metaOf(err)).toEqual({stage: 'init', region: "CN"});
 
             // 🛡️ Noise Removal: The factory internals (define.ts) must be sliced off.
             expect(topFrame).not.toContain("define.ts");

packages/core/src/define.ts

@@ -52,10 +52,10 @@ export function That<const M extends ErrorMap>(
                 Error.captureStackTrace(this, _caller);
             }
 
-            with<Me extends Record<string, unknown>>(options?: ErrorOptions, meta?: Me) {
+            with<Mt extends Record<string, unknown>>(options?: ErrorOptions, meta?: Mt) {
                 this.cause = options?.cause;
                 this[MetaField] = {...this[MetaField], ...meta};
-                return this as this & DefinedError<Extract<keyof M, string>, ExtractPayload<M[Extract<keyof M, string>]>, Record<string, unknown> & Me>;
+                return this as DefinedError<Code, Payload, Mt>;
             }
 
             is<K extends string, S extends ErrorSpec>(errorCase: ErrorCase<K, S>): this is DefinedError<K, ExtractPayload<S>> {

packages/core/src/types.ts

@@ -15,7 +15,7 @@ export interface DefinedError<
     readonly [CodeField]: Code;
     [MetaField]: Meta;
 
-    with<Me extends Record<string, unknown>>(options?: ErrorOptions, meta?: Me): this & DefinedError<Code, Payload, Meta & Me>;
+    with<Me extends Record<string, unknown>>(options?: ErrorOptions, meta?: Me): DefinedError<Code, Payload, Meta & Me>;
 
     is<K extends string, S extends ErrorSpec>(errorCase: ErrorCase<K, S>): this is DefinedError<K, ExtractPayload<S>>;
 }