Fix type errors in observers and add to pyright checked set

Group three co-assigned fields (_start_frame_id, _start_frame_arrival_ns,
_start_wall_clock) into a single _StartFrameInfo dataclass. This makes
the "always set together" invariant structural rather than implicit, and
fixes the incorrect str | None annotation on _start_frame_id (Frame.id
is int).

pyrightconfig.json

@@ -6,7 +6,8 @@
     "src/pipecat/clocks",
     "src/pipecat/metrics",
     "src/pipecat/transcriptions",
-    "src/pipecat/frames"
+    "src/pipecat/frames",
+    "src/pipecat/observers"
   ],
   "exclude": [
     "**/*_pb2.py",