chore: Follow pydantic convention to make field_validator a public method

PiperOrigin-RevId: 802861986

src/google/adk/agents/base_agent.py

@@ -504,7 +504,7 @@ class BaseAgent(BaseModel):
 
   @field_validator('name', mode='after')
   @classmethod
-  def __validate_name(cls, value: str):
+  def validate_name(cls, value: str):
     if not value.isidentifier():
       raise ValueError(
           f'Found invalid agent name: `{value}`.'

src/google/adk/agents/llm_agent.py

@@ -505,7 +505,7 @@ class LlmAgent(BaseAgent):
 
   @field_validator('generate_content_config', mode='after')
   @classmethod
-  def __validate_generate_content_config(
+  def validate_generate_content_config(
       cls, generate_content_config: Optional[types.GenerateContentConfig]
   ) -> types.GenerateContentConfig:
     if not generate_content_config: