fix: disable Gunicorn in telemetry tests to fix multi-process telemetry collection

Telemetry tests use an OTLP collector that expects single-process
telemetry spans. Gunicorn's multi-process architecture spawns multiple
workers, each with separate telemetry instrumentation, preventing the
test collector from capturing all spans.

This commit adds LLAMA_STACK_DISABLE_GUNICORN environment variable
support and sets it in telemetry test configuration to ensure
single-process Uvicorn is used during tests while maintaining
production multi-process behavior.

Fixes failing tests:
- test_streaming_chunk_count
- test_telemetry_format_completeness

tests/integration/telemetry/conftest.py

@@ -30,6 +30,7 @@ def telemetry_test_collector():
             "OTEL_EXPORTER_OTLP_PROTOCOL": "http/protobuf",
             "OTEL_BSP_SCHEDULE_DELAY": "200",
             "OTEL_BSP_EXPORT_TIMEOUT": "2000",
+            "LLAMA_STACK_DISABLE_GUNICORN": "true",  # Disable multi-process for telemetry collection
         }
 
         previous_env = {key: os.environ.get(key) for key in env_overrides}