Fix PDF MIME type for LiteLLM models

[yacinebouaouni]

Problem

The "pdf" format is currently used as the MIME type in models/lite_llm.py, causing Vertex AI to return a 400 Bad Request error due to an unsupported mimeType. For example:
litellm.exceptions.BadRequestError: litellm.BadRequestError: Error code: 400 - {'error': {'message': 'litellm.BadRequestError: VertexAIException BadRequestError - {\n "error": {\n "code": 400,\n "message": "Unable to submit request because it has a mimeType parameter with value pdf, which is not supported. Update the mimeType and try again. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini",\n "status": "INVALID_ARGUMENT"\n }\n}\n. Received Model Group=gemini-2.5-flash\nAvailable Model Group Fallbacks=None', 'type': None, 'param': None, 'code': '400'}}
`

Solution

This PR updates the MIME type to a supported format (e.g., application/pdf) in the relevant sections of the code.

Fixes #2430

Validation

• All unit tests (via pytest ./tests/unittests) now pass successfully.
• The fix has been tested end-to-end, and the operation that previously failed now completes without errors. (screenshots below)

Screenshots

The agent code

Test with adk eb

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