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/ext/extension-api/src/contributors/tools.rs

@@ -1,15 +0,0 @@
-use codex_tools::JsonToolOutput;
-use codex_tools::ToolCall;
-use codex_tools::ToolExecutor;
-
-/// Model-facing output returned by extension-owned tools.
-pub type ExtensionToolOutput = JsonToolOutput;
-
-/// Thin alias for extension-owned executable tools.
-///
-/// Extensions implement the shared `ToolExecutor<ToolCall>` contract directly;
-/// the marker keeps contributor signatures readable while preserving one
-/// executable-tool abstraction across host and extension tools.
-pub trait ExtensionToolExecutor: ToolExecutor<ToolCall, Output = ExtensionToolOutput> {}
-
-impl<T> ExtensionToolExecutor for T where T: ToolExecutor<ToolCall, Output = ExtensionToolOutput> {}