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/src/protocol/v2/tests.rs

@@ -3144,6 +3144,7 @@ fn core_turn_item_into_thread_item_converts_supported_variants() {
         app_name: Some("Calendar".to_string()),
         action_name: Some("create_event".to_string()),
         plugin_id: Some("sample@test".to_string()),
+        read_only_hint: Some(true),
         status: CoreMcpToolCallStatus::InProgress,
         result: None,
         error: None,
@@ -3167,6 +3168,7 @@ fn core_turn_item_into_thread_item_converts_supported_variants() {
             }),
             mcp_app_resource_uri: Some("app://connector".to_string()),
             plugin_id: Some("sample@test".to_string()),
+            read_only_hint: Some(true),
             result: None,
             error: None,
             duration_ms: None,
@@ -3184,6 +3186,7 @@ fn core_turn_item_into_thread_item_converts_supported_variants() {
         app_name: None,
         action_name: None,
         plugin_id: None,
+        read_only_hint: Some(false),
         status: CoreMcpToolCallStatus::Completed,
         result: Some(CallToolResult {
             content: vec![json!({"type": "text", "text": "ok"})],
@@ -3206,6 +3209,7 @@ fn core_turn_item_into_thread_item_converts_supported_variants() {
             app_context: None,
             mcp_app_resource_uri: None,
             plugin_id: None,
+            read_only_hint: Some(false),
             result: Some(Box::new(McpToolCallResult {
                 content: vec![json!({"type": "text", "text": "ok"})],
                 structured_content: Some(json!({"ok": true})),
@@ -3234,6 +3238,7 @@ fn mcp_tool_call_app_context_serializes_connector_id() {
         }),
         mcp_app_resource_uri: Some("app://connector".to_string()),
         plugin_id: None,
+        read_only_hint: Some(false),
         result: None,
         error: None,
         duration_ms: None,
@@ -3257,6 +3262,7 @@ fn mcp_tool_call_app_context_serializes_connector_id() {
             },
             "mcpAppResourceUri": "app://connector",
             "pluginId": null,
+            "readOnlyHint": false,
             "result": null,
             "error": null,
             "durationMs": null,