Make tool executor specs mandatory (#23870)

## Why

`ToolExecutor` is the runtime contract that keeps a callable tool and
its model-visible spec together. Leaving `spec()` optional lets a
registered runtime silently omit that half of the contract, and it also
overloads a missing spec as an exposure decision for tools that should
stay dispatchable without being shown to the model.

## What

- Make `ToolExecutor::spec()` required and update core, extension, and
test tool executors to return a concrete `ToolSpec`.
- Add `ToolExposure::Hidden` for dispatch-only tools. The legacy
`shell_command` runtime in unified-exec sessions now uses that explicit
exposure instead of hiding itself by omitting a spec.
- Build MCP tool specs when `McpHandler` is constructed so invalid MCP
specs are skipped before the handler is registered.
- Keep tool planning aligned with the new contract for direct, deferred,
hidden, code-mode, dynamic, and namespaced tool paths.

## Testing

- Added tool-plan coverage that invalid MCP tool specs are not
registered.
- Updated shell-family coverage for the hidden legacy `shell_command`
runtime and the affected tool executor test fixtures.

codex-rs/ext/memories/src/tools/list.rs

@@ -43,11 +43,11 @@ where
         memory_tool_name(LIST_TOOL_NAME)
     }
 
-    fn spec(&self) -> Option<ToolSpec> {
-        Some(memory_function_tool::<ListArgs, ListMemoriesResponse>(
+    fn spec(&self) -> ToolSpec {
+        memory_function_tool::<ListArgs, ListMemoriesResponse>(
             LIST_TOOL_NAME,
             "List immediate files and directories under a path in the Codex memories store.",
-        ))
+        )
     }
 
     async fn handle(

codex-rs/ext/memories/src/tools/read.rs

@@ -42,11 +42,11 @@ where
         memory_tool_name(READ_TOOL_NAME)
     }
 
-    fn spec(&self) -> Option<ToolSpec> {
-        Some(memory_function_tool::<ReadArgs, ReadMemoryResponse>(
+    fn spec(&self) -> ToolSpec {
+        memory_function_tool::<ReadArgs, ReadMemoryResponse>(
             READ_TOOL_NAME,
             "Read a Codex memory file by relative path, optionally starting at a 1-indexed line offset and limiting the number of lines returned.",
-        ))
+        )
     }
 
     async fn handle(

codex-rs/ext/memories/src/tools/search.rs

@@ -51,11 +51,11 @@ where
         memory_tool_name(SEARCH_TOOL_NAME)
     }
 
-    fn spec(&self) -> Option<ToolSpec> {
-        Some(memory_function_tool::<SearchArgs, SearchMemoriesResponse>(
+    fn spec(&self) -> ToolSpec {
+        memory_function_tool::<SearchArgs, SearchMemoriesResponse>(
             SEARCH_TOOL_NAME,
             "Search Codex memory files for substring matches, optionally normalizing separators or requiring all query substrings on the same line or within a line window.",
-        ))
+        )
     }
 
     async fn handle(