Simplify tool executor and registry plumbing (#22636)

## Why

The tool runtime path still had a typed output associated type on
`ToolExecutor`, plus a core-only `RegisteredTool` adapter and
extension-only executor aliases. That made every new shared tool runtime
carry extra adapter plumbing before it could participate in core
dispatch, extension tools, hook payloads, telemetry, and model-visible
spec generation.

This PR moves output erasure to the shared executor boundary so core and
extension tools can use the same execution contract directly.

## What Changed

- Changed `codex_tools::ToolExecutor` to return `Box<dyn ToolOutput>`
instead of an associated `Output` type.
- Removed the extension-specific `ExtensionToolExecutor` /
`ExtensionToolOutput` aliases and exposed `ToolExecutor<ToolCall>` plus
`ToolOutput` through `codex-extension-api`.
- Reworked core tool registration around `CoreToolRuntime` and
`ToolRegistry::from_tools`, removing the extra `RegisteredTool` /
`ToolRegistryBuilder` layer.
- Consolidated model-visible spec planning and registry construction in
`core/src/tools/spec_plan.rs`, including deferred tool search and
code-mode-only filtering.
- Added `ToolOutput` helpers for post-tool-use hook ids and inputs so
MCP, unified exec, extension, and other boxed outputs preserve the same
hook payload behavior.
- Updated core handlers, memories tools, and the related
registry/spec/router tests to use the simplified contract.

## Test Coverage

- Updated coverage for tool spec planning, registry lookup, deferred
tool search registration, extension tool routing, post-tool-use hook
payloads, dispatch tracing, guardian output extraction, and memories
extension tool execution.

codex-rs/tools/src/tool_executor.rs

@@ -36,8 +36,6 @@ impl ToolExposure {
 /// top without reopening the spec/runtime split.
 #[async_trait::async_trait]
 pub trait ToolExecutor<Invocation>: Send + Sync {
-    type Output: ToolOutput + 'static;
-
     /// The concrete tool name handled by this runtime instance.
     fn tool_name(&self) -> ToolName;
 
@@ -53,5 +51,8 @@ pub trait ToolExecutor<Invocation>: Send + Sync {
         false
     }
 
-    async fn handle(&self, invocation: Invocation) -> Result<Self::Output, FunctionCallError>;
+    async fn handle(
+        &self,
+        invocation: Invocation,
+    ) -> Result<Box<dyn ToolOutput>, FunctionCallError>;
 }