tests: add async timeout/retry coverage and clarify per-request overrides

[lucasalencarxisto-stack]

Note: This PR overlaps with #2615 / #2616.
If those land first, I’m happy to close this one.
The goal here is simply to improve async timeout/retry coverage and clarify per-request overrides.

Summary

This PR improves the test suite for timeouts and retries by adding clearer
async coverage and detailed assertions for per-request overrides.

Changes

• Added test_timeouts_async.py with:
  • Verification that per-request timeout overrides the client default.
  • Inverse scenario where smaller per-request timeout triggers TimeoutException.
  • Normalization helper to handle float, dict, or httpx.Timeout.
• Enhanced comments/docstrings across test_timeouts.py and test_retries.py.
• Patched OPENAI_API_KEY with a dummy value during tests.

Why

• Prevent regressions in timeout handling between sync/async clients.
• Ensure correct behavior when Retry-After headers are present (sync + async).
• Make tests easier to maintain by documenting intent and expected flow.

Notes

• Avoided UnboundLocalError in async handlers by removing inline type hints.
• Test names and comments follow a given / when / then style for clarity.

[lucasalencarxisto-stack]

Hi team 👋

This PR addresses a test issue I encountered while working with timeouts/retries:

• Fixed UnboundLocalError in async timeout tests by removing inline type hints and ensuring httpx is imported only at the top level.
• Added explicit async coverage to confirm per-request timeouts override client defaults, and that smaller per-request values properly raise TimeoutException.
• Improved retry tests with clearer comments and docstrings, especially around handling Retry-After headers and server error retries.

The main goal is to make timeout/retry behavior more robust against regressions and easier to understand for future contributors.

Happy to adjust anything if needed 🙏