feat: add optional you.com search integration

[mouse-value-add]

Summary

This adds You.com as an optional search engine provider, following the exact same SearchEngine pattern used by Exa, SerpApi, Tavily, and ArXiv.

You.com offers both API-key-authenticated and keyless search — the keyless tier works immediately with no API key or signup, making it the easiest way to get started with a web search backend.

What changed

• Added ms_agent/tools/search/youcom/ with search.py, schema.py, http.py following the Tavily provider pattern exactly
• Registered YOCOM in SearchEngineType enum and ENGINE_TOOL_NAMES
• Added dispatch in search_engine.py, websearch_tool.py (both get_search_engine_class and get_search_engine), and search_request.py
• Updated .env.example with YDC_API_KEY (documented as optional)
• Updated projects/deep_research/conf.yaml with a commented-out youcom config example
• Updated projects/fin_research/README.md with You.com as another search engine option

Setup

Three ways to use it:

1. Keyless (no config needed): set engine: youcom in conf.yaml
2. API key: set YDC_API_KEY in .env (get one at https://you.com/platform/api-keys)
3. Programmatic: get_search_engine('youcom') or YouSearch()

Why optional

• No new dependencies (stdlib only: urllib, json)
• Existing Exa/SerpApi/Tavily/ArXiv behavior unchanged
• When YDC_API_KEY is unset, it uses the keyless free tier transparently
• API errors raise typed exceptions with structured fields (status, code, retry_after)

Validation

All 4 new files compile cleanly. Unit tests verified:

• Schema serialization (YouSearchRequest.to_api_body)
• Result parsing (YouSearchResult.to_list with empty/valid responses)
• HTTP error classification (quota 429, auth 401/403, transport failures)
• YouSearch keyless mode vs keyed mode (_search_url, _headers)
• build_request_from_args and get_tool_definition
• Enum registration (SearchEngineType.YOCOM, ENGINE_TOOL_NAMES['youcom'])

I kept this optional, following the existing provider pattern. Happy to adjust the shape if you prefer a different abstraction.