Initial Checks

• I confirm that I'm using the latest version of Pydantic AI

  I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue
  To pick up a draggable item, press the space bar. While dragging, use the arrow keys to move the item. Press space again to drop the item in its new position, or press escape to cancel.

Description

When manually passing a token limit to the agent that's too restrictive and causes the model run to fail, the error we raise is of the wrong type

from pydantic_ai import Agent

Agent("google-gla:gemini-2.5-pro-preview-05-06", model_settings=dict(max_tokens=5)).run_sync("write a haiku")

yields

UnexpectedModelBehavior: Content field missing from Gemini response, body: (...)

instead of UsageLimitExceeded

Example Code

from pydantic_ai import Agent
Agent("google-gla:gemini-2.5-pro-preview-05-06", model_settings=dict(max_tokens=5)).run_sync("write a haiku")

Python, Pydantic AI & LLM client version

Python 3.12.4
Pydantic 0.3.1
google-gla:gemini-2.5-pro-preview-05-06