feat: accept injected httpx2 clients

[dsfaccini]

Draft / RFC — not for direct merge. A minimal, duck-typed demonstration to help move #2680 forward. This repo mirrors an internal source of truth via Copybara and doesn't merge external PRs directly, so the value here is making the change concrete, not landing it.

httpx2 is Pydantic's actively-maintained, API-compatible fork of httpx. Because it lives in a separate import namespace, httpx2.* classes fail isinstance(x, httpx.*). Handing the SDK an httpx2 client via HttpOptions(httpx_async_client=...) hits two walls today:

• Wall 1 — rejected at construction. HttpOptions.httpx_async_client / httpx_client are typed via the HttpxClient / HttpxAsyncClient aliases in generated google/genai/types.py; with arbitrary_types_allowed=True, Pydantic emits an is_instance_of(httpx.AsyncClient) check, so an httpx2 client raises ValidationError.
• Wall 2 — response not processed. Hand-written isinstance(x, httpx.Response/Headers) checks and the retry-exception tuple in errors.py / _api_client.py don't recognize httpx2 objects. The async success path is the worst case: its status_code == 200 early-return is nested inside isinstance(response, httpx.Response), so every async httpx2 response falls through to "Unsupported response type".

Behind a guarded import httpx2 (a no-op when httpx2 isn't installed), this widens the runtime checks:

• Hand-written (the real, stick-able change): errors.py raise_for_response + raise_for_async_response; _api_client.py HttpResponse.__init__ headers check, _iter_response_stream + _aiter_response_stream (validity guard + iterator selection), and the retry_args retry predicate — each via one widened tuple per concept.
• Generated (cannot land as-is): types.py widens the HttpxClient / HttpxAsyncClient aliases to Union[httpx.*, httpx2.*]. This file is generated ("DO NOT EDIT"), so the real Wall 1 fix must happen in the generator template; it's included here only so the demonstration runs end-to-end.

Adds a test (guarded by pytest.importorskip("httpx2")) covering both walls with real httpx2 objects, and declares httpx2 as a test dependency (py3.10+, which required bumping the anyio / idna pins it needs).

Scope / known limitation: this covers the models.generate_content(_stream) path (sync + async, streaming + non-streaming). The injected client is also forwarded into the experimental client.agents / interactions / webhooks / triggers surfaces (the _gaos sub-SDK), which still couple to httpx and are not handled here — that's a separate, larger generated-code change. Deliberately does not touch the default-client base classes or the client-arg filter.

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