feat: search_tool

[apanasenko-oai]

Why We Did This

• The goal is to reduce MCP tool context pollution by not exposing the full MCP tool list up front
• It forces an explicit discovery step (search_tool_bm25) so the model narrows tool scope before making MCP calls, which helps relevance and lowers prompt/tool clutter.

What It Changed

• Added a new experimental feature flag search_tool in core/src/features.rs:90 and core/src/features.rs:430.
• Added config/schema support for that flag in core/config.schema.json:214 and core/config.schema.json:1235.
• Added BM25 dependency (bm25) in Cargo.toml:129 and core/Cargo.toml:23.
• Added new tool handler search_tool_bm25 in core/src/tools/handlers/search_tool_bm25.rs:18.
• Registered the handler and tool spec in core/src/tools/handlers/mod.rs:11 and core/src/tools/spec.rs:780 and core/src/tools/spec.rs:1344.
• Extended ToolsConfig to carry search_tool enablement in core/src/tools/spec.rs:32 and core/src/tools/spec.rs:56.
• Injected dedicated developer instructions for tool-discovery workflow in core/src/codex.rs:483 and core/src/codex.rs:1976, using core/templates/search_tool/developer_instructions.md:1.
• Added session state to store one-shot selected MCP tools in core/src/state/session.rs:27 and core/src/state/session.rs:131.
• Added filtering so when feature is enabled, only selected MCP tools are exposed on the next request (then consumed) in core/src/codex.rs:3800 and core/src/codex.rs:3843.
• Added E2E suite coverage for enablement/instructions/hide-until-search/one-turn-selection in core/tests/suite/search_tool.rs:72, core/tests/suite/search_tool.rs:109, core/tests/suite/search_tool.rs:147, and core/tests/suite/search_tool.rs:218.
• Refactored test helper utilities to support config-driven tool collection in core/tests/suite/tools.rs:281.

Net Behavioral Effect

• With search_tool off: existing MCP behavior (tools exposed normally).
• With search_tool on: MCP tools start hidden, model must call search_tool_bm25, and only returned selected_tools are available for the next model call.

[chatgpt-codex-connector]

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