fix: raise ValueError when MIME type cannot be determined for file_uri

[giulio-leone]

Summary

Fixes #5184

When file_data.file_uri has no explicit MIME type and no recognizable extension, the LiteLLM integration previously fell back silently to application/octet-stream via _DEFAULT_MIME_TYPE. This broke providers that validate MIME types (e.g. Vertex AI for GCS URIs) without any warning.

Changes

1. Raise ValueError instead of silent fallback (_get_content)

• Removed the _DEFAULT_MIME_TYPE fallback assignment when MIME type cannot be determined.
• A ValueError with actionable guidance is now raised only when a file block would actually be constructed (i.e. the provider requires it).
• Provider-specific text fallback paths (anthropic, vertex_ai non-gemini, openai/azure non-file-id URIs) continue to work without requiring MIME type — they never construct a file block.

2. Propagate model= in recursive _content_to_message_param() call

• The recursive call for mixed function_response + file content now forwards model=model, fixing incorrect provider detection (e.g. Vertex AI Gemini vs non-Gemini).

Tests

• Updated test_content_to_message_param_user_message_file_uri_without_mime_type → expects ValueError
• Updated test_get_content_file_uri_default_mime_type → expects ValueError
• Added test_get_content_file_uri_no_mime_text_fallback_still_works — verifies text fallback works without MIME
• Added test_content_to_message_param_recursive_model_propagation — verifies model forwarding keeps file blocks for vertex_ai/gemini
• Added test_content_to_message_param_recursive_model_propagation_fallback — verifies text fallback for vertex_ai non-gemini

All 248 tests pass (3 new, 2 updated).

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