Fix Gemini Live local VAD by sending correct activity events to server

When Gemini Live was configured with local VAD (server-side VAD disabled),
the service was listening for the wrong frame types and not sending
ActivityStart/ActivityEnd events to the server. Now it listens for
VADUserStartedSpeakingFrame/VADUserStoppedSpeakingFrame and sends the
appropriate activity signals when local VAD is in use.

Also removes the unnecessary local SileroVADAnalyzer from server-side VAD
examples and adds a new 26a example demonstrating local VAD configuration.

examples/foundational/26-gemini-live.py

@@ -10,8 +10,6 @@ import os
 from dotenv import load_dotenv
 from loguru import logger
 
-from pipecat.audio.vad.silero import SileroVADAnalyzer
-from pipecat.audio.vad.vad_analyzer import VADParams
 from pipecat.frames.frames import LLMRunFrame
 from pipecat.pipeline.pipeline import Pipeline
 from pipecat.pipeline.runner import PipelineRunner
@@ -20,7 +18,6 @@ from pipecat.processors.aggregators.llm_context import LLMContext
 from pipecat.processors.aggregators.llm_response_universal import (
     AssistantTurnStoppedMessage,
     LLMContextAggregatorPair,
-    LLMUserAggregatorParams,
     UserTurnStoppedMessage,
 )
 from pipecat.runner.types import RunnerArguments
@@ -71,16 +68,8 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
             },
         ],
     )
-    user_aggregator, assistant_aggregator = LLMContextAggregatorPair(
-        context,
-        user_params=LLMUserAggregatorParams(
-            # Set stop_secs to something roughly similar to the internal setting
-            # of the Multimodal Live api, just to align events. This doesn't
-            # really matter because we can only use the Multimodal Live API's
-            # phrase endpointing, for now.
-            vad_analyzer=SileroVADAnalyzer(params=VADParams(stop_secs=0.5))
-        ),
-    )
+    # Server-side VAD is enabled by default; no local VAD is added.
+    user_aggregator, assistant_aggregator = LLMContextAggregatorPair(context)
 
     pipeline = Pipeline(
         [