feat(ai): route xAI models through Responses and default to Grok 4.6 (#8124)

* feat(ai): route xAI models through Responses and default to Grok 4.6

Send built-in xAI catalog models through the Responses API with
store: false and include reasoning.encrypted_content so encrypted
reasoning is requested and replayed, including future models.dev
additions. Thinking levels come from models.dev reasoning_options
(grok-4.6 exposes xhigh); models without verified options
(grok-build-0.1) keep reasoning always on and never send unsupported
"none"/"minimal" efforts. Narrow the xAI provider to the Responses API,
send pi's runtime User-Agent on xAI requests for provider-side
attribution, and make Grok 4.6 the default xAI model.

* fix(ai): drop duplicate xAI encrypted-reasoning tests

Azure already covers the shared replay path; keep the xAI-specific include-without-effort case next to the existing request tests.

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Co-authored-by: Jaaneek <Jaaneek@users.noreply.github.com>