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/core/src/tools/router_tests.rs

@@ -7,7 +7,6 @@ use crate::turn_diff_tracker::TurnDiffTracker;
 use codex_extension_api::ExtensionData;
 use codex_extension_api::ExtensionRegistry;
 use codex_extension_api::ExtensionRegistryBuilder;
-use codex_extension_api::ExtensionToolExecutor;
 use codex_extension_api::ResponsesApiTool;
 use codex_extension_api::ToolCall as ExtensionToolCall;
 use codex_extension_api::ToolExecutor;
@@ -37,7 +36,7 @@ impl codex_extension_api::ToolContributor for ExtensionEchoContributor {
         &self,
         _session_store: &ExtensionData,
         _thread_store: &ExtensionData,
-    ) -> Vec<Arc<dyn ExtensionToolExecutor>> {
+    ) -> Vec<Arc<dyn ToolExecutor<ExtensionToolCall>>> {
         vec![Arc::new(ExtensionEchoExecutor)]
     }
 }
@@ -46,8 +45,6 @@ struct ExtensionEchoExecutor;
 
 #[async_trait::async_trait]
 impl ToolExecutor<ExtensionToolCall> for ExtensionEchoExecutor {
-    type Output = codex_tools::JsonToolOutput;
-
     fn tool_name(&self) -> ToolName {
         ToolName::namespaced("extension/", "echo")
     }
@@ -78,14 +75,14 @@ impl ToolExecutor<ExtensionToolCall> for ExtensionEchoExecutor {
     async fn handle(
         &self,
         call: ExtensionToolCall,
-    ) -> Result<Self::Output, codex_tools::FunctionCallError> {
+    ) -> Result<Box<dyn codex_tools::ToolOutput>, codex_tools::FunctionCallError> {
         let arguments: serde_json::Value =
             serde_json::from_str(call.function_arguments()?).expect("test arguments should parse");
-        Ok(codex_tools::JsonToolOutput::new(json!({
+        Ok(Box::new(codex_tools::JsonToolOutput::new(json!({
             "arguments": arguments,
             "callId": call.call_id,
             "ok": true,
-        })))
+        }))))
     }
 }