Fix gemini weak model

[Dharunika-07-N]

Added graceful fallback for weak model when not found

Changes

• Modified get_weak_model() in models.py:
  • Added a try-except block around the creation of the weak model.
  • If the weak model is missing (404), a warning is printed and self.weak_model defaults to the main model.
• Ensures that Aider continues to work even if the weak model is unavailable.

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