fix(genai): Fix outdated doc for max_output_tokens default value (#1209)

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## Description

The current documentation incorrectly states that the default value of
the `max_output_tokens` parameter is `64`.
In reality, the default varies depending on the model.

This update clarifies that the default is model-specific and adds a
reference to the official Gemini API documentation for model-specific
limits.

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🐛 Bug Fix


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Author: tawAsh1

libs/genai/langchain_google_genai/_common.py

@@ -52,7 +52,8 @@ class _BaseGoogleGenerativeAI(BaseModel):
 
     max_output_tokens: Optional[int] = Field(default=None, alias="max_tokens")
     """Maximum number of tokens to include in a candidate. Must be greater than zero.
-    If unset, will default to ``64``."""
+    If unset, will use the model's default value, which varies by model.
+    See https://ai.google.dev/gemini-api/docs/models for model-specific limits."""
 
     n: int = 1
     """Number of chat completions to generate for each prompt. Note that the API may