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/core/src/tools/spec_plan.rs

@@ -72,6 +72,7 @@ use codex_tools::ToolEnvironmentMode;
 use codex_tools::ToolExecutor;
 use codex_tools::ToolName;
 use codex_tools::ToolOutput;
+use codex_tools::ToolSearchInfo;
 use codex_tools::ToolSpec;
 use codex_tools::UnifiedExecShellMode;
 use codex_tools::can_request_original_image_detail;
@@ -937,6 +938,10 @@ impl ToolExecutor<ToolInvocation> for MultiAgentV2NamespaceOverride {
         self.handler.supports_parallel_tool_calls()
     }
 
+    fn search_info(&self) -> Option<ToolSearchInfo> {
+        self.handler.search_info()
+    }
+
     async fn handle(
         &self,
         invocation: ToolInvocation,
@@ -950,10 +955,6 @@ impl CoreToolRuntime for MultiAgentV2NamespaceOverride {
         self.handler.matches_kind(payload)
     }
 
-    fn search_info(&self) -> Option<crate::tools::tool_search_entry::ToolSearchInfo> {
-        self.handler.search_info()
-    }
-
     fn create_diff_consumer(
         &self,
     ) -> Option<Box<dyn crate::tools::registry::ToolArgumentDiffConsumer>> {