Add api_host and timeout to XaiProvider

[colesmcintosh]

• Part of xAI: review support for newer SDK request and provider options #5662 (Group A)

Forwards the xAI SDK client's api_host (gateway / regional / proxy setups) and timeout (client-level default) options through XaiProvider, so users don't have to drop down to a prebuilt xai_client for common deployment ergonomics. Both map to real SDK constructor parameters (api_host: str, timeout: float | None), and unset options are omitted so the SDK's own defaults apply.

This combines what were originally two separate PRs (#5742 for api_host and #5743 for timeout); since both rewrote the same __init__ overloads and as written were mutually exclusive, shipping them together avoids a merge conflict. #5743 is closed in favor of this one.

Checklist

• Any AI generated code has been reviewed line-by-line by the human PR author, who stands by it.
• No breaking changes in accordance with the version policy.
• PR title is fit for the release changelog.

[colesmcintosh]

The failing test on 3.x (lowest-versions) checks are unrelated to this change. They're caused by aiohttp 3.14.0 (published to PyPI on 2026-06-01) removing aiohttp.streams.AsyncStreamReaderMixin, which vcrpy still imports — so the lowest-direct resolver pulls the new aiohttp and the whole suite errors at collection. Tracked in #5751, with the fix in #5752 (Pin aiohttp below 3.14 for vcrpy).

This branch is already up to date with main, so once #5752 merges I'll re-merge main here to pick up the constraint and the lowest-versions matrix should go green. All other CI jobs pass.