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/analytics/src/analytics_client_tests.rs

@@ -4536,6 +4536,7 @@ async fn turn_event_counts_completed_tool_items() {
         }),
         mcp_app_resource_uri: None,
         plugin_id: Some("sample@test".to_string()),
+        read_only_hint: None,
         result: None,
         error: None,
         duration_ms,