feat(tools): add MiniMax as bundled web search provider

Add native MiniMax Search integration via their Coding Plan search API
(POST /v1/coding_plan/search). This brings MiniMax in line with Brave,
Kimi, Grok, Gemini, and other providers that already have bundled web
search support.

- Implement WebSearchProviderPlugin with caching, credential resolution,
  and trusted endpoint wrapping
- Support both global (api.minimax.io) and CN (api.minimaxi.com)
  endpoints, inferred from explicit region config, model provider base
  URL, or minimax-portal OAuth base URL
- Prefer MINIMAX_CODE_PLAN_KEY over MINIMAX_API_KEY in credential
  fallback, matching existing repo precedence
- Accept SecretRef objects for webSearch.apiKey (type: [string, object])
- Register in bundled registry, provider-id compat map, and fast-path
  plugin id list with full alignment test coverage
- Add unit tests for endpoint/region resolution and edge cases

Closes #47927
Related #11399

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/plugins/contracts/bundled-web-search.minimax.contract.test.ts

@@ -0,0 +1,3 @@
+import { describeBundledWebSearchFastPathContract } from "../../../test/helpers/plugins/bundled-web-search-fast-path-contract.js";
+
+describeBundledWebSearchFastPathContract("minimax");