Expose managed WebMCP policy through the app server (#42823)

## What changed

- Parse the optional `[browser_use].allow_webmcp` boolean from managed requirements and preserve requirement-layer precedence.
- Return the policy as `browserUse.allowWebmcp` from `configRequirements/read`, including explicit `true`, explicit `false`, and `null` when omitted.
- Update the generated JSON and TypeScript protocol schemas.

## Testing

- Cover parsing, invalid values, layered overrides, and app-server response serialization.

GitOrigin-RevId: a3fd7d1a6f838bb3ab0ac2a644409101a2338ca6

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

@@ -215,6 +215,12 @@
             "null"
           ]
         },
+        "allowWebmcp": {
+          "type": [
+            "boolean",
+            "null"
+          ]
+        },
         "defaultOriginPolicy": {
           "anyOf": [
             {