feat(google): support Cloud Code /v1internal:*generateContent (Antigravity, Gemini CLI)

[drakonkat]

Summary

Support native Google inference requests rerouted through pxpipe from clients and providers using the Google wire format:

• /v1/models/{model}:generateContent and /v1beta/models/{model}:streamGenerateContent (both methods on both versions), alongside existing Google AI Studio gateway paths.
• Cloud Code /v1internal:generateContent and :streamGenerateContent, including request and generateContentRequest envelopes.
• Preserve Google OAuth/API credentials without injecting Anthropic credentials. Skip Cloud Code countTokens probes.
• GOOGLE_UPSTREAM overrides the shared upstream for Google inference; retain explicit Cloudflare gateway routing and daily Cloud Code host detection.
• Merge current main, retaining model opt-out, thought signatures, and Cloud Code response accounting.
• Fix Windows TypeScript build entry resolution using Node path utilities.

This routes Google-format requests; it does not translate OpenAI Responses or Chat Completions payloads into Google payloads.

Validation

• TypeScript typecheck passed.
• Full Vitest suite: 1,224 tests passed.
• After final upstream-precedence adjustment: 23 routing tests passed.
• Build and bundled CLI version smoke check passed on Windows / Node 24.
• GitHub reports MERGEABLE with no conflicts at e342aeb.
• GitHub CI requires maintainer approval for this fork: https://github.com/teamchong/pxpipe/actions/runs/34201881084
• Live Google/OAuth requests were not exercised; routing tests use stubbed fetch.