make httpx connect timout configurable

[rasdani]

Description

don't hardcode httpx connect timeout for OAI or any other client anymore.
helps with vLLM or cluster flakiness.

related: PrimeIntellect-ai/prime-rl#2017

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Low Risk
Low risk: adds a new config field and wires it into httpx.Timeout without changing request logic beyond the connect-timeout value.

Overview
Makes the HTTP connect timeout configurable via ClientConfig.connect_timeout and EndpointClientConfig.connect_timeout (default 5.0).

Updates HTTP client construction to use config.connect_timeout instead of a hardcoded connect timeout when building the httpx.AsyncClient timeout settings.

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