fix(core): default approval behavior for mcp missing annotations (#15519)

- Changed `requires_mcp_tool_approval` to apply MCP spec defaults when
annotations are missing.
- Unannotated tools now default to:
  - `readOnlyHint = false`
  - `destructiveHint = true`
  - `openWorldHint = true`
- This means unannotated MCP tools now go through approval/ARC
monitoring instead of silently bypassing it.
- Explicitly read-only tools still skip approval unless they are also
explicitly marked destructive.

**Previous behavior**
Failed open for missing annotations, which was unsafe for custom MCP
tools that omitted or forgot annotations.

---------

Co-authored-by: colby-oai <228809017+colby-oai@users.noreply.github.com>

codex-rs/core/tests/common/apps_test_server.rs

@@ -185,6 +185,11 @@ impl Respond for CodexAppsJsonRpcResponder {
                             {
                                 "name": "calendar_create_event",
                                 "description": "Create a calendar event.",
+                                "annotations": {
+                                    "readOnlyHint": false,
+                                    "destructiveHint": false,
+                                    "openWorldHint": false
+                                },
                                 "inputSchema": {
                                     "type": "object",
                                     "properties": {
@@ -209,6 +214,9 @@ impl Respond for CodexAppsJsonRpcResponder {
                             {
                                 "name": "calendar_list_events",
                                 "description": "List calendar events.",
+                                "annotations": {
+                                    "readOnlyHint": true
+                                },
                                 "inputSchema": {
                                     "type": "object",
                                     "properties": {
@@ -241,6 +249,9 @@ impl Respond for CodexAppsJsonRpcResponder {
                         tools.push(json!({
                             "name": format!("calendar_timezone_option_{index}"),
                             "description": format!("Read timezone option {index}."),
+                            "annotations": {
+                                "readOnlyHint": true
+                            },
                             "inputSchema": {
                                 "type": "object",
                                 "properties": {