feat(adk): native Gemini factory for Format=Gemini proxies

[tbolis-at-mulesoft]

Closes #691

Summary

• donkey.adk.gemini("gemini-2.5-flash", **kw) and the module-level donkey_kit.integrations.adk.gemini return ADK's native google.adk.models.Gemini, bound to a Format=Gemini proxy. The shared DonkeyAsyncClient is injected through HttpOptions.httpx_async_client (BG §1.8). gemini_connection_kwargs() is the manual-equivalent escape hatch. model() is unchanged (LiteLLM).
• Core (§1.1, still framework-free):
  • core/transport.py reads the requested model from the Gemini URL path (models/<m>:generateContent|streamGenerateContent) when the body has none.
  • core/lastcall.py parses Gemini usageMetadata, both buffered and SSE.
  • Result: spans, usage, per-run correlation and last_call all work, and gemini() records no conformance exemption. The ADK KNOWN_LIMITATIONS rows are now scoped to model().
• Live-verified 2026-09-29 against ddk-gemini-inbound (§0.3):
  • x-goog-api-key coexists with client_id/client_secret.
  • SSE streaming and tool calling work.
  • Bad secret → 401, which classifies as AuthError. Unknown model → Google's 404, which classifies as UpstreamRequestError, via classify(exc.response). The native APIError is not wrapped.
  • The gateway ignores a body model field.
  • Redacted fixtures were added under tests/fixtures/anypoint/gemini_inbound/.
• verify_frameworks.py has a new adk.gemini row. The live check runs when DONKEY_GEMINI_PROXY_URL is set; it reported VERIFIED (LIVE) here.

Docs (ddk-docs-sync)

• docs/verified-apis.md: the §2 Gemini row was re-probed and "no SDK adapter" removed; §8 has a new ADK-native-Gemini row, VERIFIED (LIVE). Maintainer sign-off on that row = approving this PR.
• frameworks/adk.mdx has a new Native Gemini section: the three forms, the manual equivalent, base_url, which last_call fields stay None, reading last_call in after_model_callback, classify(exc.response), and one Donkey per event loop.
• Also updated:
  • frameworks/index.mdx (Format mapping and injection table)
  • feature-overview, telemetry, reference/last-call, and examples/{adk,gemini}
  • both READMEs' exemption rows, ARCHITECTURE.md and CONTRIBUTING.md
  • python/examples/adk, the fixture README and the docstrings
  • website/public (regenerated)
• examples/general/gateway-identity.mdx was left unchanged: it is captured demo output and is still accurate for model().

Test plan

• pytest -q: 780 passed, 10 skipped
• mypy, ruff check ., lint-imports (5/5 contracts kept)
• verify_frameworks.py --only adk: both rows signature-confirmed offline; --only adk.gemini --live → VERIFIED (LIVE)
• website: next build passes (61 pages)

Post-deploy steps

• Acceptance repo: add test_adk_builds_a_native_gemini_model and its coverage.toml entry once a release containing gemini() is published (follow-up: Donkey-Development-Kit/donkey-development-kit-acceptance#10).
• Known limitation, not new: the shared httpx pool is bound to its first event loop, so reusing one Donkey across asyncio.run calls fails. It is documented.
• Rotate the ddk-gemini-inbound consumer pair used for the live run.