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

[drakonkat]

Summary

Antigravity and Gemini CLI talk to Google through the Cloud Code endpoint
https://cloudcode-pa.googleapis.com/v1internal:streamGenerateContent (and :generateContent).
The body is an envelope { model, project, request: { contents, ... } } around a normal
generateContent request.

pxpipe only matched /google-ai-studio/(v1|v1beta)/models/{model}:(stream)generateContent,
so the Cloud Code route fell through to the default (Anthropic) upstream and got no compression.

Changes

• src/core/google.ts: GOOGLE_INTERNAL_ROUTE / isGoogleInternalPath, unwrapGoogleRequest
  (request / generateContentRequest keys), readGoogleInternalModel. transformGoogleGenerateContent
  recurses into the inner request when it sees the envelope and preserves the outer fields.
• src/core/proxy.ts: googleUpstream config (default https://cloudcode-pa.googleapis.com),
  /v1internal:*generateContent counted as a Google route, model read from the envelope,
  :countTokens probe skipped and x-api-key stripped on the internal route.
• src/node.ts / src/worker.ts: GOOGLE_UPSTREAM env (falls back to PXPIPE_UPSTREAM), help text.
• tests/google.test.ts, tests/gateway.test.ts: envelope unwrap, route detection, upstream selection, header stripping.
• CHANGELOG.md: Unreleased entry.

Test plan

• pnpm typecheck
• pnpm vitest run — 77 files / 1210 tests pass
• New tests for the envelope and the internal route
• Live run through Antigravity (needs a Cloud Code OAuth session) — only gemini-3.6-flash / gemini-3.7-flash have measured profiles, other models pass through as unsupported_model