Fix type errors in serializers and add to pyright checked set

Moves src/pipecat/serializers into pyright's include list. Narrows
self._params to each subclass's InputParams in exotel, vonage, plivo,
twilio, genesys, and telnyx. In protobuf.py, renames the reassigned
frame local to avoid clobbering its Frame type and silences two dynamic
attribute accesses on the generated frames_pb2 module.

Also aligns telnyx and plivo hangup validation with twilio: if
auto_hang_up=True (the default) but required credentials are missing,
__init__ now raises ValueError instead of silently logging a warning
at call-end time. Previously a misconfigured serializer would construct
fine and fail to hang up the call later, leaving a phantom billable
session.

pyrightconfig.json

@@ -11,6 +11,7 @@
     "src/pipecat/observers",
     "src/pipecat/pipeline",
     "src/pipecat/runner",
+    "src/pipecat/serializers",
     "src/pipecat/tests",
     "src/pipecat/transcriptions",
     "src/pipecat/turns",
@@ -21,7 +22,6 @@
     "src/pipecat/adapters",
     "src/pipecat/audio",
     "src/pipecat/processors",
-    "src/pipecat/serializers",
     "src/pipecat/services",
     "src/pipecat/transports",
     "tests"