Move tool search metadata onto ToolExecutor (#25684)

Deferred tools need to be searchable even when they are not implemented
inside `codex-core`. Extension-provided tools can be registered for
later discovery, but the search metadata path was still owned by
core-specific runtime hooks, which meant the shared `ToolExecutor`
abstraction could not describe how a deferred extension tool should
appear in `tool_search`.

## Changes

- Move `ToolSearchEntry` and `ToolSearchInfo` into `codex-tools` and
re-export them from the shared tools crate.
- Add a default `ToolExecutor::search_info` implementation that derives
loadable tool-search metadata from function and namespace specs.
- Forward search metadata through extension adapters and exposure
overrides while keeping custom search text/source metadata for dynamic,
MCP, and multi-agent tools.
- Remove the old core-local `tool_search_entry` module now that search
metadata lives with the shared executor APIs.

## Testing

- Added `deferred_extension_tools_are_discoverable_with_tool_search`
coverage in `core/src/tools/spec_plan_tests.rs`.

codex-rs/tools/src/lib.rs

@@ -17,6 +17,7 @@ mod tool_discovery;
 mod tool_executor;
 mod tool_output;
 mod tool_payload;
+mod tool_search;
 mod tool_spec;
 
 pub use code_mode::augment_tool_spec_for_code_mode;
@@ -97,6 +98,9 @@ pub use tool_executor::ToolExposure;
 pub use tool_output::JsonToolOutput;
 pub use tool_output::ToolOutput;
 pub use tool_payload::ToolPayload;
+pub use tool_search::ToolSearchEntry;
+pub use tool_search::ToolSearchInfo;
+pub use tool_search::default_tool_search_text;
 pub use tool_spec::ResponsesApiWebSearchFilters;
 pub use tool_spec::ResponsesApiWebSearchUserLocation;
 pub use tool_spec::ToolSpec;