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/core/src/tools/code_mode/execute_handler.rs

@@ -94,8 +94,8 @@ impl ToolExecutor<ToolInvocation> for CodeModeExecuteHandler {
         ToolName::plain(PUBLIC_TOOL_NAME)
     }
 
-    fn spec(&self) -> Option<ToolSpec> {
-        Some(self.spec.clone())
+    fn spec(&self) -> ToolSpec {
+        self.spec.clone()
     }
 
     async fn handle(

codex-rs/core/src/tools/code_mode/wait_handler.rs

@@ -47,8 +47,8 @@ impl ToolExecutor<ToolInvocation> for CodeModeWaitHandler {
         ToolName::plain(WAIT_TOOL_NAME)
     }
 
-    fn spec(&self) -> Option<ToolSpec> {
-        Some(create_wait_tool())
+    fn spec(&self) -> ToolSpec {
+        create_wait_tool()
     }
 
     async fn handle(