log plugin MCP server names (#26002)

## Summary
- emit the plugin capability summary's exact MCP server names in
`codex_plugin_used`

## Test
- `just test -p codex-analytics`
- `just test -p codex-core
explicit_plugin_mentions_track_plugin_used_analytics`
- `just fix -p codex-analytics`

codex-rs/analytics/src/analytics_client_tests.rs

@@ -2828,6 +2828,7 @@ fn plugin_used_event_serializes_expected_shape() {
                 "mcp_server_count": 2,
                 "connector_ids": ["calendar", "drive"],
                 "product_client_id": originator().value,
+                "mcp_server_names": ["mcp-1", "mcp-2"],
                 "thread_id": "thread-3",
                 "turn_id": "turn-3",
                 "model_slug": "gpt-5"