Include suggestion IDs in plugin install metadata (#39765)

## What changed

- Add `suggestion_id` to plugin install elicitation metadata, using the same
  value as the elicitation request ID and plugin-install analytics event.
- Leave connector install metadata unchanged by omitting the field.

## Testing

- Cover plugin and connector metadata serialization.
- Verify that remote plugin install metadata and analytics carry matching
  suggestion IDs.

GitOrigin-RevId: 05dd1d210e2a82857094f09d27ecba79ee460b7e

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -2935,7 +2935,9 @@ mod tests {
             turn_id: Some("turn_123".to_string()),
             server_name: "codex_apps".to_string(),
             request: v2::McpServerElicitationRequest::Form {
-                meta: None,
+                meta: Some(json!({
+                    "suggestion_id": "request_plugin_install_install-github"
+                })),
                 message: "Allow this request?".to_string(),
                 requested_schema,
             },
@@ -2954,7 +2956,9 @@ mod tests {
                     "turnId": "turn_123",
                     "serverName": "codex_apps",
                     "mode": "form",
-                    "_meta": null,
+                    "_meta": {
+                        "suggestion_id": "request_plugin_install_install-github"
+                    },
                     "message": "Allow this request?",
                     "requestedSchema": {
                         "type": "object",