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>

docs/reference/secretref-credential-surface.md

@@ -47,6 +47,7 @@ Scope intent:
 - `plugins.entries.moonshot.config.webSearch.apiKey`
 - `plugins.entries.perplexity.config.webSearch.apiKey`
 - `plugins.entries.firecrawl.config.webSearch.apiKey`
+- `plugins.entries.minimax.config.webSearch.apiKey`
 - `plugins.entries.tavily.config.webSearch.apiKey`
 - `tools.web.search.apiKey`
 - `gateway.auth.password`

docs/reference/secretref-user-supplied-credentials-matrix.json

@@ -540,6 +540,13 @@
       "secretShape": "secret_input",
       "optIn": true
     },
+    {
+      "id": "plugins.entries.minimax.config.webSearch.apiKey",
+      "configFile": "openclaw.json",
+      "path": "plugins.entries.minimax.config.webSearch.apiKey",
+      "secretShape": "secret_input",
+      "optIn": true
+    },
     {
       "id": "plugins.entries.moonshot.config.webSearch.apiKey",
       "configFile": "openclaw.json",