fix: wrap mid-stream transport errors

[xianjianlf2]

Summary

• translate read timeouts raised while consuming sync and async streams into APITimeoutError
• translate other httpx2.TransportError failures into APIConnectionError
• preserve the original transport exception as __cause__
• keep wrapping scoped to response-byte iteration so parsing and SDK-originated errors remain unchanged

The stream is not retried after events have already been yielded; this PR restores the SDK exception hierarchy without replaying a partially consumed response.

Fixes #1919.

Testing

• UV_FROZEN=1 UV_SKIP_WHEEL_FILENAME_CHECK=1 ./scripts/test tests/test_streaming.py -q
  • Python 3.10 / Pydantic v2: 27 passed
  • Python 3.10 / Pydantic v1: 27 passed
  • Python 3.14 / Pydantic v2: 27 passed
  • MCP v2 suite: 39 passed
• ./scripts/lint
  • Ruff, dependency caps, Pyright, Mypy, and import check passed
• git diff --check