Expose MCP read-only hints in tool call items (#36055)

## What changed

- Propagate MCP tool `readOnlyHint` annotations through tool-call start and
  completion events.
- Include the optional hint in persisted thread history and app-server
  `mcpToolCall` items, preserving compatibility when the annotation is absent.
- Document that the hint describes tool capability, not the outcome of a
  particular invocation.

## Testing

- Cover read-only and write-capable tools in live events and persisted rollout
  data.
- Verify the hint survives thread reads and resumes for both in-progress and
  completed MCP calls.

GitOrigin-RevId: dddfe905146075e5137a1094da485b86b99807f0

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

@@ -1385,6 +1385,12 @@
                 "null"
               ]
             },
+            "readOnlyHint": {
+              "type": [
+                "boolean",
+                "null"
+              ]
+            },
             "result": {
               "anyOf": [
                 {