add ToolSearchToolset and ToolProxyToolset for dynamic tool discovery

[longcw]

When agents have many tools (10+), loading all tool definitions into the LLM context upfront degrades accuracy and wastes tokens. This PR adds two toolsets that enable on-demand tool discovery and loading.

ToolSearchToolset — exposes a tool_search function. Matched tools are added directly to the LLM's tool list on the next turn, so the model uses native tool calls. Simpler for the model.

ToolProxyToolset — exposes exactly two fixed tools: tool_search (returns schemas) and call_tool (executes by name). The tool list remains unchanged, preserving prompt cache across turns for providers like OpenAI and Anthropic.

Both support:

• Nested toolsets (MCPToolset), standalone tools, ProviderTool, and RawFunctionTool
• Atomic toolset loading: if a tool picked belongs to a nested toolset, the whole toolset is loaded.
• Pluggable search via SearchStrategy protocol
• Built-in keywod and BM25 search strategy

need to merge #5138 first

References

• ya-agent-sdk Tool Search / Tool Proxy
• OpenAI Tool Search
• Anthropic Advanced Tool Use

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.