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/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -8100,12 +8100,89 @@
     "HookHandlerType": {
       "enum": [
         "command",
+        "mcpTool",
         "prompt",
         "agent"
       ],
       "type": "string"
     },
     "HookMetadata": {
+      "oneOf": [
+        {
+          "properties": {
+            "async": {
+              "default": false,
+              "type": "boolean"
+            },
+            "command": {
+              "type": "string"
+            },
+            "handlerType": {
+              "enum": [
+                "command"
+              ],
+              "type": "string"
+            }
+          },
+          "required": [
+            "command",
+            "handlerType"
+          ],
+          "type": "object"
+        },
+        {
+          "properties": {
+            "handlerType": {
+              "enum": [
+                "mcpTool"
+              ],
+              "type": "string"
+            },
+            "server": {
+              "type": "string"
+            },
+            "tool": {
+              "type": "string"
+            }
+          },
+          "required": [
+            "handlerType",
+            "server",
+            "tool"
+          ],
+          "type": "object"
+        },
+        {
+          "properties": {
+            "handlerType": {
+              "enum": [
+                "prompt"
+              ],
+              "type": "string"
+            }
+          },
+          "required": [
+            "handlerType"
+          ],
+          "title": "PromptHookMetadata",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "handlerType": {
+              "enum": [
+                "agent"
+              ],
+              "type": "string"
+            }
+          },
+          "required": [
+            "handlerType"
+          ],
+          "title": "AgentHookMetadata",
+          "type": "object"
+        }
+      ],
       "properties": {
         "additionalContextLimit": {
           "description": "Configured `additionalContext` spill threshold. `null` uses 2,500 tokens; `0` disables spilling.",
@@ -8116,12 +8193,6 @@
             "null"
           ]
         },
-        "command": {
-          "type": [
-            "string",
-            "null"
-          ]
-        },
         "currentHash": {
           "type": "string"
         },
@@ -8135,17 +8206,6 @@
         "eventName": {
           "$ref": "#/definitions/HookEventName"
         },
-        "executionMode": {
-          "allOf": [
-            {
-              "$ref": "#/definitions/HookExecutionMode"
-            }
-          ],
-          "default": "sync"
-        },
-        "handlerType": {
-          "$ref": "#/definitions/HookHandlerType"
-        },
         "isManaged": {
           "type": "boolean"
         },
@@ -8190,7 +8250,6 @@
         "displayOrder",
         "enabled",
         "eventName",
-        "handlerType",
         "isManaged",
         "key",
         "source",