feat(tools): add tool_search for deferred MCP tool loading (#1176)

* feat(tools): add tool_search for deferred MCP tool loading

When multiple MCP servers are enabled, total tool count can exceed 30-50,
causing context bloat and degraded tool selection accuracy. This adds a
deferred tool loading mechanism controlled by `tool_search.enabled` config.

- Add ToolSearchConfig with single `enabled` field
- Add DeferredToolRegistry with regex search (select:, +keyword, keyword)
- Add tool_search tool returning OpenAI-compatible function JSON
- Add DeferredToolFilterMiddleware to hide deferred schemas from bind_tools
- Add <available-deferred-tools> section to system prompt
- Enable MCP tool_name_prefix to prevent cross-server name collisions
- Add 34 unit tests covering registry, tool, prompt, and middleware

* fix: reset stale deferred registry and bump config_version

- Reset deferred registry upfront in get_available_tools() to prevent
  stale tool entries when MCP servers are disabled between calls
- Bump config_version to 2 for new tool_search config field

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

* fix(tests): mock get_app_config in prompt section tests for CI

CI has no config.yaml, causing TestDeferredToolsPromptSection to fail
with FileNotFoundError. Add autouse fixture to mock get_app_config.

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

backend/packages/harness/deerflow/mcp/tools.py

@@ -53,7 +53,7 @@ async def get_mcp_tools() -> list[BaseTool]:
         if oauth_interceptor is not None:
             tool_interceptors.append(oauth_interceptor)
 
-        client = MultiServerMCPClient(servers_config, tool_interceptors=tool_interceptors)
+        client = MultiServerMCPClient(servers_config, tool_interceptors=tool_interceptors, tool_name_prefix=True)
 
         # Get all tools from all servers
         tools = await client.get_tools()