fix isXrpcErrorOf mismatch test to actually test a mismatch

The 'different error code' test asserted a MATCHING code returns true
(with an unrelated method as witness), contradicting its own name and
leaving the code-mismatch path untested. Split into two tests: a real
mismatch (approveJoinRequest declares both names, so no as-any cast
needed) asserting false, and the non-XRPC case.

Found by a roast pass over the error-matching changes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/strings/__tests__/errors.test.ts

@@ -73,14 +73,17 @@ describe('isXrpcErrorOf', () => {
     ).toBe(true)
   })
 
-  it('does not match a different error code or a non-XRPC value', () => {
+  it('does not match a different error code', () => {
     expect(
       isXrpcErrorOf(
-        chat.bsky.convo.getMessages,
+        chat.bsky.group.approveJoinRequest,
         lexError(400, 'InvalidConvo'),
-        'InvalidConvo',
+        'InsufficientRole',
       ),
-    ).toBe(true)
+    ).toBe(false)
+  })
+
+  it('does not match a non-XRPC value', () => {
     expect(
       isXrpcErrorOf(
         chat.bsky.convo.acceptConvo,