Add MCP tool handler support to the hooks engine (#38705)

## What changed

- Discover synchronous `mcp_tool` hook handlers and invoke their configured MCP server and tool through a supplied executor.
- Expand nested hook-event placeholders in MCP tool inputs while preserving JSON types, and process tool output through the existing hook output contract.
- Represent hook details as handler-specific metadata in `hooks/list`, including MCP server and tool fields, and show those details in the TUI hooks browser.
- Skip unsupported `SessionEnd` MCP hooks and runtimes without MCP invocation support with startup warnings.

## Testing

- Cover argument expansion, missing placeholders, MCP invocation and hook decisions, discovery warnings, `hooks/list` metadata, and TUI rendering.

GitOrigin-RevId: 295b845471fe92bd7ad7cd272fbcd2c3713912e0

codex-rs/core/src/session/mod.rs

@@ -4172,6 +4172,7 @@ async fn build_hooks_config(
         plugin_hook_load_warnings,
         shell_program: hook_shell_program,
         shell_args: hook_shell_argv,
+        mcp_executor: None,
     }
 }