feat(web): add Xiaomi MiMo native search

[h3c-hexin]

Summary

• Enable Xiaomi MiMo's documented web_search Chat Completions plugin for mimo-v2.5-pro and mimo-v2.5 on official routes.
• Force a bounded search, preserve structured annotation metadata, and require verifiable citations before the backend considers the attempt usable.
• Keep ultraspeed, speech/TTS models, and custom-compatible endpoints fail-closed.

Refs #5681 (partial).

No-Issue: partial delivery tracked by #5681; this slice must not close the umbrella issue.
Related: #5682.

Testing

• cargo fmt --all -- --check
• cargo test -p codewhale-config mimo_native_search_is_exact_to_documented_chat_models -- --nocapture
• cargo test -p codewhale-tui mimo_ -- --nocapture — 29 passed, including both new adapter tests
• cargo check -p codewhale-config -p codewhale-tui
• git diff --check

Safety boundary

• The search request is non-streaming, forces at most the requested result count, disables thinking, and caps completion output at 2,048 tokens.
• Only structured provider annotations become citations. An answer without verifiable citations remains unusable and falls through to configured/local search.
• Domain-constrained requests use configured/local fallback because the MiMo plugin does not expose a proven server-side domain filter.

Honesty boundary

This exact upstream commit was verified with deterministic request/response tests, not a funded live MiMo account. No credentials, release, tag, deployment, or publication action is part of this PR. The workspace-wide baseline findings are recorded on #5682.