Pin fastapi to Python 3.14 explicitly to match fastapi_startkit

fastapi already inherited the repo-wide 3.14 default, but declaring it
explicitly makes the fastapi vs fastapi_startkit comparison unambiguous:
both configs now state Python 3.14, so any benchmark delta reflects the
frameworks, not the runtime.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: tmgbedu

python/fastapi/config.yaml

@@ -7,3 +7,6 @@ framework:
     - hypercorn
     - daphne
     - granian
+
+language:
+  version: 3.14