fix(openai): omit timeout from client_params when not set by user

[Mohammad Mohtashim (keenborder786)]

When request_timeout is None (the default), we passed timeout=None to the OpenAI SDK, which interprets it as "disable all timeouts" — bypassing the built-in 600s default and causing infinite hangs on network issues.

Now we omit the timeout key entirely when unset, so the SDK default applies. Aligns with langchain-google-genai's existing approach.

Fixes #35597

[codspeed-hq]

Merging this PR will degrade performance by 12.03%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

❌ 3 regressed benchmarks
⏩ 37 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	test_init_time	3.2 ms	3.7 ms	-12.03%
❌	WallTime	test_init_time	121.5 ms	137.7 ms	-11.75%
❌	WallTime	test_init_time	3.2 ms	3.7 ms	-11.54%

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_{Comparing keenborder786:fix/update_default_value (d784e35) with master (90d015c)}

Footnotes

1. 37 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. ↩