[pull] main from google:main

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Created by pull[bot] (v2.0.0-alpha.4)

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Summary by cubic

Centralizes Google Cloud and Agent Platform setup into a single guide and updates docs to link to it, reducing duplication and simplifying setup. Adds Apigee OpenAI-compatible client example and expands BigQuery Agent Analytics Java guidance.

• New Features

  • Added “Connect to Google Cloud and Agent Platform” guide covering setup, auth (user credentials, service account, express mode, agent identity), and model discovery; added to nav.
  • Apigee: Introduced OpenAI-compatible CompletionsHTTPClient with example for /chat/completions proxies.
  • BigQuery Agent Analytics: Added Java content formatter example; bumped Java support note to v1.5.0.

• Refactors

  • Consolidated model/auth instructions across pages (Gemini, Agent Platform models, Cloud Run, tutorials, eval criteria, memory, sessions, agent/skills registry, grounding, streaming, trace) to link to the new guide.
  • Clarified env vars, introducing GOOGLE_GENAI_USE_ENTERPRISE (equivalent to GOOGLE_GENAI_USE_VERTEXAI).
  • Minor copy and link cleanups for consistency.

^{Written for commit 7d3fe44. Summary will update on new commits.}