Closes #103378

What Problem This Solves

Fixes an issue where users combining allowed_x_handles and excluded_x_handles, or supplying more than 20 handles, would send an invalid x_search request to xAI and receive a provider-side failure.

Why This Change Was Made

The xAI plugin now publishes the provider's 20-handle limit in the tool schema and validates both the limit and mutual exclusion at the plugin-owned input boundary before network I/O. The core tool registry and provider-routing behavior are unchanged.

User Impact

Invalid X Search handle filters now fail immediately with a clear tool-input error. Models also see the constraints in the schema, reducing invalid billed-tool attempts.

Evidence

• Official xAI contract: X Search filters and fixed-source SDK validation.
• pnpm test extensions/xai/x-search.test.ts extensions/xai/index.test.ts — 61/61 passed on Node 22.23.1.
• pnpm check:changed --timed — extension and extension-test typechecks, changed-file lint, import-cycle checks, and repository policy guards passed.
• Fresh structured autoreview — clean; no accepted or actionable findings.
• Blacksmith Testbox infrastructure stopped during checkout before executing the focused command: run 29069861898. The local run above is the documented unavailable-provider fallback; PR CI remains the authoritative hosted gate.
• No paid live xAI request was made; the invalid shapes are rejected deterministically before provider I/O.

Release Note Context

• xAI X search filters: reject mutually exclusive allow/exclude handle filters and enforce xAI's 20-handle limit before provider requests. ([Bug]: x_search accepts invalid handle filter combinations #103378, fix(xai): reject invalid x_search handle filters #103393)

AI-assisted: implemented and reviewed with Codex. I inspected the code paths, tests, documentation, and upstream xAI contract and understand the change.