fix: handle JSON.stringify edge cases in stringifyUnknown

Wrap JSON.stringify in try/catch to gracefully handle circular
references and BigInt values. Falls back to String() which is
always safe. Add tests for both edge cases.

Addresses Cursor BugBot feedback on PR #259.

Author: BYK

src/lib/errors.ts

@@ -330,7 +330,13 @@ export function stringifyUnknown(value: unknown): string {
     return value.message;
   }
   if (value && typeof value === "object") {
-    return JSON.stringify(value);
+    // JSON.stringify can throw on circular references or BigInt values.
+    // Fall back to String() which is always safe.
+    try {
+      return JSON.stringify(value);
+    } catch {
+      return String(value);
+    }
   }
   return String(value);
 }

test/lib/errors.test.ts

@@ -291,6 +291,21 @@ describe("stringifyUnknown", () => {
     expect(stringifyUnknown(true)).toBe("true");
     expect(stringifyUnknown(0)).toBe("0");
   });
+
+  test("falls back to String() for circular references", () => {
+    const circular: Record<string, unknown> = { name: "loop" };
+    circular.self = circular;
+    // Should not throw — falls back to String() which returns [object Object]
+    expect(() => stringifyUnknown(circular)).not.toThrow();
+    expect(stringifyUnknown(circular)).toBe("[object Object]");
+  });
+
+  test("falls back to String() for BigInt values", () => {
+    const obj = { count: BigInt(42) };
+    // JSON.stringify throws on BigInt — should fall back gracefully
+    expect(() => stringifyUnknown(obj)).not.toThrow();
+    expect(stringifyUnknown(obj)).toBe("[object Object]");
+  });
 });
 
 describe("formatError", () => {