feat(ai): add Google Vertex AI provider

[Taitranz]

Summary

• Adds @effect/ai-google-vertex — Google Vertex AI provider
• Reuses prompt conversion, response parsing, and tool logic from @effect/ai-google (same Gemini wire format)
• Adds GCP-specific Bearer token auth, project/location URL construction, and v1/v1beta1 API version support
• Exports prepareMessages, makeResponse, makeStreamResponse, prepareTools, jsonSchemaToOpenApiSchema, and resolveFinishReason from @effect/ai-google/GoogleLanguageModel to enable reuse

Split from #6165 per @IMax153's request to target v4.

Test plan

• Verify package builds successfully
• Verify language model implementation works against Google Vertex AI API
• Verify shared exports from @effect/ai-google/GoogleLanguageModel are correctly exposed