feat: dynamic tools injection (#9539)

## Summary
Add dynamic tool injection to thread startup in API v2, wire dynamic
tool calls through the app server to clients, and plumb responses back
into the model tool pipeline.

### Flow (high level)
- Thread start injects `dynamic_tools` into the model tool list for that
thread (validation is done here).
- When the model emits a tool call for one of those names, core raises a
`DynamicToolCallRequest` event.
- The app server forwards it to the client as `item/tool/call`, waits
for the client’s response, then submits a `DynamicToolResponse` back to
core.
- Core turns that into a `function_call_output` in the next model
request so the model can continue.

### What changed
- Added dynamic tool specs to v2 thread start params and protocol types;
introduced `item/tool/call` (request/response) for dynamic tool
execution.
- Core now registers dynamic tool specs at request time and routes those
calls via a new dynamic tool handler.
- App server validates tool names/schemas, forwards dynamic tool call
requests to clients, and publishes tool outputs back into the session.
- Integration tests

codex-rs/app-server/README.md

@@ -114,7 +114,20 @@ Start a fresh thread when you need a new Codex conversation.
     "cwd": "/Users/me/project",
     "approvalPolicy": "never",
     "sandbox": "workspaceWrite",
-    "personality": "friendly"
+    "personality": "friendly",
+    "dynamicTools": [
+        {
+            "name": "lookup_ticket",
+            "description": "Fetch a ticket by id",
+            "inputSchema": {
+                "type": "object",
+                "properties": {
+                    "id": { "type": "string" }
+                },
+                "required": ["id"]
+            }
+        }
+    ],
 } }
 { "id": 10, "result": {
     "thread": {