Add Python POC for client-executed MCP tool search over Responses

[adityachaudhry]

Summary

Add a one-file Python POC for client-executed MCP tool search through the Copilot SDK with an OpenAI BYOK provider using the Responses API.

The session starts with a deferred openai_docs MCP server. When the model calls tool_search, the SDK's tool_search_tool override queries that server, adds newly discovered client tools through session.tools.set, and returns tool_references. Subsequent tool calls are routed back to MCP. Direct CLI-managed MCP execution is rejected and detected so the sample cannot silently take a different path. A redacted wire audit checks that successful Responses requests contain tool_search_call and tool_search_output with matching call_id and execution: "client".

No SDK source changes were needed: the current tool-search override, tool references, and experimental session.tools.set RPC already support the sequence. The POC uses only the public OpenAI Docs MCP server and reads its API key from OPENAI_API_KEY.

Verification

• Repeated live runs with gpt-5.6-sol, OpenAI Responses, and Copilot CLI 1.0.89-1 completed with a final answer, including two fresh-session repetitions and a run after review fixes.
• Wire traces showed the initial /v1/responses request offering native tool_search; the next request contained tool_search_call and client tool_search_output, whose call_id matched and whose tools were absent initially; later rounds contained function_call and function_call_output.
• Python unit suite: 577 passed (python --ignore=python/e2e) with a fresh CLI cache path.
• Ruff lint and format checks, py_compile, and git diff --check passed.

Scope and compatibility

This is an exploratory sample. session.tools.set is experimental and was verified against CLI 1.0.89-1. The runtime currently sends store: false and reconstructs input on each Responses round rather than using previous_response_id; the tool-search and function-call item sequence still matches the direct Responses POC.

Relates to #1896.