fix: reuse google auth per provider instance

[heiwen]

Background

@ai-sdk/google-vertex used a module-level GoogleAuth cache in the shared Node auth helper. That cache was invalidated by object reference equality, which caused unnecessary GoogleAuth recreation in two cases:

1. Multiple provider instances with equivalent googleAuthOptions would recreate auth when requests alternated between them.
2. When googleAuthOptions was omitted, generateAuthToken(options || {}) created a new empty object on every call, so auth could be recreated on every request even for a single provider.

Because GoogleAuth caches the resolved auth client on the instance, recreating it discards warm auth state and can add avoidable auth overhead.

Summary

This PR removes the module-level GoogleAuth singleton from @ai-sdk/google-vertex's shared Node auth helper and replaces it with a per-provider auth token generator.

Changes included:

• Replaced the shared generateAuthToken(options) helper with createAuthTokenGenerator(options), which creates one GoogleAuth instance and returns a reusable token generator closure.
• Updated the Node provider entry points for:
  • @ai-sdk/google-vertex
  • @ai-sdk/google-vertex/anthropic
  • @ai-sdk/google-vertex/maas
• Added regression tests covering:
  • repeated calls with omitted googleAuthOptions
  • repeated calls on a single provider instance
  • per-provider generator creation for the Node provider variants
• Added a patch changeset for @ai-sdk/google-vertex

Manual Verification

I manually verified the change by inspecting the Node auth flow and confirming the auth generator is now created once during provider construction rather than on each request path.

Concretely:

• confirmed each Node provider now creates createAuthTokenGenerator(options.googleAuthOptions) once at provider initialization
• confirmed request-time header/fetch code now calls the returned generator with no options, so the undefined -> {} invalidation path is gone
• confirmed the shared helper no longer stores module-level auth state

I also verified behavior through automated coverage:

• pnpm --filter @ai-sdk/google-vertex test:node
• pnpm type-check:full

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Future Work

A follow-up PR could evaluate whether the Edge auth path should also cache tokens or auth state more aggressively, since it uses a different implementation and is not addressed here.

Related Issues

Fixes #14101