Include 'timed out' message in async timeout exceptions

[Kludex]

Closes #742.

The sync backend raises timeout exceptions carrying socket.timeout's "timed out" message, but the async backends wrap anyio's bare TimeoutError (and trio's TooSlowError), which have empty messages - so httpx2.ReadTimeout printed nothing in async tracebacks and logs.

map_exceptions now supplies "timed out" when mapping an exception with an empty message to a TimeoutException subclass, making sync and async consistent.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[github-actions]

Docs preview: https://7c6a7ee0-httpx2-docs.pydantic.workers.dev

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing timeout-message (44f44d8) with main (0c5012e)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}