Fix third-party Anthropic provider connection tests

[Linlearner]

Summary\n- Use the user-entered model mapping when testing Anthropic third-party providers, instead of always falling back to short aliases like sonnet.\n- Classify HTTP 503 / temporarily unavailable provider responses explicitly, so the UI no longer shows the generic "unexpected error" message.\n- Add regression coverage for model-mapping connection tests and 503 classification.\n\n## Verification\n- npx --yes tsx --test src/__tests__/unit/provider-key-lifecycle.test.ts src/__tests__/unit/error-classifier.test.ts\n- git diff --cached --check\n\n## Notes\n- npm ci currently fails because package.json and package-lock.json are out of sync on main (missing @electron/windows-sign transitive entries), so I used temporary npx runners for the focused tests.\n- Full tsc --noEmit could not complete without the project dependency tree installed; the failure was missing dependencies/types, not this patch.

[vercel]

Someone is attempting to deploy a commit to the op7418's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Linlearner]

Additional verification after review:

• The connection-test path now sends the user-entered model mapping (for example a full Claude model ID) instead of falling back to the short alias sonnet. This is the functional fix for third-party Anthropic-compatible providers that reject short aliases.
• The normal runtime path already resolves short aliases through role model mappings; this patch closes the missing connection-test path.
• Focused regression tests pass: npx --yes tsx --test src/__tests__/unit/provider-key-lifecycle.test.ts src/__tests__/unit/error-classifier.test.ts.
• Current Vercel failure is fork deployment authorization, not a test/build failure.