feat(python): add aiohttp auto-detection for async HTTP client

[jsklan]

Description

WIP — This PR modifies the seed fixture output directly. Generator changes to produce this output will follow.

Auto-detect httpx_aiohttp at runtime and use it for the async HTTP client when available, falling back to plain httpx.AsyncClient.

Based on: cohere-ai/cohere-python#739

Changes Made

• Add _make_default_async_client() helper in client.py that tries httpx_aiohttp.HttpxAiohttpClient first, falling back to httpx.AsyncClient
• Add _default_clients.py module with DefaultAioHttpClient and DefaultAsyncHttpxClient convenience classes (SDK-appropriate defaults)
• Add optional [aiohttp] extra in pyproject.toml
• Add pytest-based tests with @pytest.mark.aiohttp marker and auto-skip logic in conftest.py
• Add CI two-pass testing: standard tests then pytest -m aiohttp after installing the extra
• Increase vitest timeouts for basic generation ETE tests from 60s → 180s to match the internal cliV2.generate process timeout (fixes test-ete CI flake where Python remote generation occasionally exceeds 60s)
• Updated README.md generator (if applicable)

Updates since last revision

• Rebased onto main to resolve mergeable status
• Fixed importlib.reload bug in test_default_aiohttp_client_raises_without_package: the cleanup reload was inside the mock.patch.dict context manager, which left seed._default_clients polluted with the error-raising class after the mock restored sys.modules. Moved the reload outside the with block so the module is properly restored.
• Fixed test-ete CI timeout: the vitest it() timeout for the basic generation > python test was 60s, but cliV2.generate() uses an internal process timeout of 180s. Python remote generation took ~68s in CI, exceeding the vitest timeout. Increased both python and go basic generation test timeouts to 180_000ms to match.

Testing

• pytest -m "not aiohttp" — 6 tests pass (fallback behavior without httpx_aiohttp)
• pip install httpx-aiohttp && pytest -m "aiohttp" — 3 tests pass (autodetect with httpx_aiohttp)
• test-ete CI check now passes with increased timeouts
• Update generator to produce this output
• Run seed tests to validate generator matches fixture

Review checklist for humans

• Verify _make_default_async_client handles both httpx_aiohttp present and absent cases correctly
• Confirm importlib.reload fix: the reload at line 97 of test_aiohttp_autodetect.py is now outside the with mock.patch.dict(...) block — this prevents module pollution when httpx_aiohttp is actually installed
• Note: test mocking uses mock.patch.dict(sys.modules, {"httpx_aiohttp": None}) to simulate missing package — verify this doesn't leak across tests
• Confirm the 180s vitest timeout increase for basic generation tests is acceptable (matches internal cliV2.generate timeout; the outer describe block already has a 300s timeout)

Link to Devin session: https://app.devin.ai/sessions/2562e5b9bd6a4be7a5756c141751c323

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[claude]

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