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/tool_executor.rs

@@ -1,6 +1,7 @@
 use crate::FunctionCallError;
 use crate::ToolName;
 use crate::ToolOutput;
+use crate::ToolSearchInfo;
 use crate::ToolSpec;
 
 /// Controls where a tool is exposed to the model.
@@ -13,7 +14,9 @@ pub enum ToolExposure {
     Direct,
 
     /// Register this tool for later discovery, but omit it from the initial
-    /// model-visible tool list.
+    /// model-visible tool list. Deferred tools must provide search metadata via
+    /// [`ToolExecutor::search_info`]. The default implementation derives
+    /// metadata from function and namespace specs.
     Deferred,
 
     /// Include this tool in the initial model-visible tool list only.
@@ -48,6 +51,11 @@ pub trait ToolExecutor<Invocation>: Send + Sync {
         ToolExposure::Direct
     }
 
+    fn search_info(&self) -> Option<ToolSearchInfo> {
+        let spec = self.spec();
+        ToolSearchInfo::from_tool_spec(&self.tool_name(), spec, /*source_info*/ None)
+    }
+
     fn supports_parallel_tool_calls(&self) -> bool {
         false
     }