Add extra comments about Google model deprecation (#4424)

Author: chenghao-mou

livekit-plugins/livekit-plugins-google/livekit/plugins/google/realtime/api_proto.py

@@ -5,17 +5,21 @@
 
 from google.genai import types
 
+# Gemini API deprecations: https://ai.google.dev/gemini-api/docs/deprecations
+# Gemini API release notes with preview deprecations: https://ai.google.dev/gemini-api/docs/changelog
+# live models: https://docs.cloud.google.com/vertex-ai/generative-ai/docs/live-api
+# VertexAI retirement: https://docs.cloud.google.com/vertex-ai/generative-ai/docs/learn/model-versions#retired-models
+# Additional references:
+# 1. https://github.com/kazunori279/adk-streaming-test/blob/main/test_report.md
 LiveAPIModels = Literal[
     # VertexAI models
-    "gemini-live-2.5-flash-native-audio",
-    "gemini-live-2.5-flash-preview-native-audio",
-    # deprecated vertexai models
-    "gemini-2.0-flash-exp",
-    "gemini-live-2.5-flash-preview-native-audio-09-2025",
+    "gemini-live-2.5-flash-native-audio",  # GA https://docs.cloud.google.com/vertex-ai/generative-ai/docs/models/gemini/2-5-flash-live-api#live-2.5-flash
+    "gemini-live-2.5-flash-preview-native-audio-09-2025",  # Public preview https://docs.cloud.google.com/vertex-ai/generative-ai/docs/models/gemini/2-5-flash-live-api#live-2.5-flash-preview
+    "gemini-live-2.5-flash-preview-native-audio",  # still works, possibly an alias, but not mentioned in any docs or changelog
     # Gemini API models
-    "gemini-2.5-flash-native-audio-preview-12-2025",
-    # deprecated Gemini API models
-    "gemini-2.5-flash-native-audio-preview-09-2025",
+    "gemini-2.5-flash-native-audio-preview-12-2025",  # https://ai.google.dev/gemini-api/docs/models#gemini-2.5-flash-live
+    "gemini-2.5-flash-native-audio-preview-09-2025",  # https://ai.google.dev/gemini-api/docs/models#gemini-2.5-flash-live
+    "gemini-2.0-flash-exp",  # still works in Gemini API but not VertexAI
 ]
 
 Voice = Literal[