test: add shared test mocks and stabilize unit tests ✨

**🧪 Tests**
- Add comprehensive messageNormalization.test.ts to explicitly test role mapping and normalization logic.
- Consolidate and move the showModels quick-pick clipboard regression into manageConfig tests and refine assertions to reduce brittleness.

**🧹 Chores**
- Introduce createMockSecrets and createMockOutputChannel helpers in src/test/utils/testMocks.ts to DRY repeated secret and output-channel mocks.
- Replace many inline secret/output-channel mocks across tests with the new helpers for consistency and clearer test setup.

**🐛 Fixes**
- Enhance client test response mock to emulate headers.get("retry-after"), statusText, and clone, improving fidelity of fetch-response tests.
- Tweak various stubs and assertions (showQuickPick item shapes, showInputBox behavior, withProgress/checkConnection flows, and assert.ok usage) to make tests more reliable.