[Refactor] Reuse shared Responses client mock in X.AI tests

[zoomote]

Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Reused the shared mockOpenAiResponsesClient factory in the X.AI provider spec.
• Preserved the X.AI-specific Responses API payloads, stream fixtures, reasoning cases, tool assertions, and error handling tests.

Why this change was made

This continues the incremental test-utils rollout after the shared helper foundations and provider batches. It removes repeated Responses client mock construction while preserving the clarity of the X.AI-specific tests.

Impact

Test-only refactor. X.AI behavior is unchanged, and future Responses API provider tests have a shared, Vitest-hoisting-safe client mock.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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