fix(google-vertex): use per-instance auth token generator to avoid cache thrashing

[Jah-yee]

Closes #14101

The previous module-level singleton used reference equality to detect
options changes, which caused two issues:

1. Multiple provider instances with equivalent googleAuthOptions would
   thrash the singleton when alternating between them
2. Omitted googleAuthOptions would create a new empty object each call,
   causing unnecessary recreation

This fix replaces the shared helper with a factory function that creates
a per-instance auth token generator. Each provider instance now gets
stable auth/client caching without requiring structural serialization.

---

Changes:

• Added factory function in
• Updated to use per-instance auth generator
• Kept backward-compatible function for other consumers

[vercel]

Additional Suggestions:

1. Test file imports _resetAuthInstance which was removed from the source module, causing the test to fail at import time.

2. Anthropic vertex provider uses deprecated generateAuthToken which creates a new GoogleAuth instance on every API request, causing a performance regression.

3. Test mocks generateAuthToken but production code now imports and uses createAuthTokenGenerator, causing all tests to fail with createAuthTokenGenerator is not a function.