app-server: add filesystem watch support (#14533)

### Summary
Add the v2 app-server filesystem watch RPCs and notifications, wire them
through the message processor, and implement connection-scoped watches
with notify-backed change delivery. This also updates the schema
fixtures, app-server documentation, and the v2 integration coverage for
watch and unwatch behavior.

This allows clients to efficiently watch for filesystem updates, e.g. to
react on branch changes.

### Testing
- exercise watch lifecycles for directory changes, atomic file
replacement, missing-file targets, and unwatch cleanup

codex-rs/app-server-protocol/schema/json/v2/FsChangedNotification.json

@@ -0,0 +1,29 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "AbsolutePathBuf": {
+      "description": "A path that is guaranteed to be absolute and normalized (though it is not guaranteed to be canonicalized or exist on the filesystem).\n\nIMPORTANT: When deserializing an `AbsolutePathBuf`, a base path must be set using [AbsolutePathBufGuard::new]. If no base path is set, the deserialization will fail unless the path being deserialized is already absolute.",
+      "type": "string"
+    }
+  },
+  "description": "Filesystem watch notification emitted for `fs/watch` subscribers.",
+  "properties": {
+    "changedPaths": {
+      "description": "File or directory paths associated with this event.",
+      "items": {
+        "$ref": "#/definitions/AbsolutePathBuf"
+      },
+      "type": "array"
+    },
+    "watchId": {
+      "description": "Watch identifier returned by `fs/watch`.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "changedPaths",
+    "watchId"
+  ],
+  "title": "FsChangedNotification",
+  "type": "object"
+}