chore: Update default and example Gemini models to new versions

Changes references from `gemini-1.5-flash` and `gemini-1.5-pro` to `gemini-2.5-flash` and `gemini-2.5-pro` in docstrings, default values, sample agents, and tests.

PiperOrigin-RevId: 805536434

Author: hangfei

contributing/samples/bigtable/agent.py

@@ -62,7 +62,7 @@
 # The variable name `root_agent` determines what your root agent is for the
 # debug CLI
 root_agent = LlmAgent(
-    model="gemini-1.5-flash",
+    model="gemini-2.5-flash",
     name="bigtable_agent",
     description=(
         "Agent to answer questions about Bigtable database tables and"

src/google/adk/models/base_llm.py

@@ -32,7 +32,7 @@ class BaseLlm(BaseModel):
   """The BaseLLM class.
 
   Attributes:
-    model: The name of the LLM, e.g. gemini-1.5-flash or gemini-1.5-flash-001.
+    model: The name of the LLM, e.g. gemini-2.5-flash or gemini-2.5-pro.
   """
 
   model_config = ConfigDict(
@@ -42,7 +42,7 @@ class BaseLlm(BaseModel):
   """The pydantic model config."""
 
   model: str
-  """The name of the LLM, e.g. gemini-1.5-flash or gemini-1.5-flash-001."""
+  """The name of the LLM, e.g. gemini-2.5-flash or gemini-2.5-pro."""
 
   @classmethod
   def supported_models(cls) -> list[str]:

src/google/adk/models/google_llm.py

@@ -58,7 +58,7 @@ class Gemini(BaseLlm):
     model: The name of the Gemini model.
   """
 
-  model: str = 'gemini-1.5-flash'
+  model: str = 'gemini-2.5-flash'
 
   retry_options: Optional[types.HttpRetryOptions] = None
   """Allow Gemini to retry failed responses.

src/google/adk/models/lite_llm.py

@@ -683,7 +683,7 @@ def _is_litellm_gemini_model(model_string: str) -> bool:
 
   Args:
     model_string: A LiteLLM model string (e.g., "gemini/gemini-2.5-pro" or
-      "vertex_ai/gemini-1.5-flash")
+      "vertex_ai/gemini-2.5-flash")
 
   Returns:
     True if it's a Gemini model accessed via LiteLLM, False otherwise