fix(client): tolerate older optional aiohttp installations (#3941)

## Summary

Fixes #3939.

When older aiohttp is installed, importing the optional adapter
currently
raises `AttributeError` for `aiohttp.SocketTimeoutError`. The SDK
catches
`ImportError`, so this breaks `import openai` even for applications
using
only the default HTTPX2 clients.

Import the required exception explicitly so the existing
unavailable-adapter
path handles its absence. No SDK version bounds or new minimum aiohttp
version checks are introduced. The `openai[aiohttp]` dependency
requirement
remains unchanged, and the optional adapter remains unavailable on the
older exception surface.

This refreshes the minimal import change previously proposed by
@marcuswood-oai in #3868, with sync/async regressions and vendor
documentation.

## Validation

- Reproduced the original bare-import failure using the unmodified
v3.18.0
  release and actual aiohttp 3.9.3.
- Added absent-aiohttp and pre-3.10 subprocess cases. The legacy case
failed
with the original `AttributeError` before the fix and passed afterward.
- Each subprocess exercises synchronous and asynchronous SDK requests
through HTTPX2 mock transports, checks the explicit unsupported-adapter
  error, and verifies legacy HTTPX was not imported.
- The regression preserves `aiohttp.client` and earlier exception
imports
  so an unrelated `ModuleNotFoundError` cannot make the old code pass.
- Targeted transport suites: 39 passed, one skipped because optional
  `botocore` is not installed. This includes a real local HTTP request
  through the supported aiohttp adapter.
- Real aiohttp 3.8.6, 3.9.3, 3.10.0, and 3.14.3 fixtures passed default
  sync/async request probes against this patch. Old versions are test
fixtures, not recommended installations or supported aiohttp transports.
- Changed tests passed Ruff lint and formatting. Existing repository
  exclusions for vendored code remain unchanged.

```shell
python -m pytest -o addopts= tests/test_httpx2.py tests/test_httpx2_base.py tests/test_httpx2_respx.py tests/test_httpx2_workload.py -q -rs
```

No dependency, generated-baseline, custom-code budget, or workflow
changes.
Full CI and maintainer review remain required.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Marcus Wood <marcuswood@openai.com>
Copilot-Session: 0857d4b0-2459-4f7b-af29-366e05d08090