feat(component,ai,gemini): implement automatic format conversion for unsupported media types (#1128)

Because

- Users had to manually convert media formats or use special syntax
(`:png`, `:pdf`, etc.) when working with formats not directly supported
by Gemini API
- The existing validation system only provided error messages suggesting
manual conversion, creating friction in the user experience
- The complex `formatSupport` struct and validation logic made the
codebase harder to maintain and understand

This commit

- Implements automatic format conversion for unsupported MIME types in
Gemini AI component:
  - **Images**: Automatically converts GIF, BMP, TIFF → PNG
  - **Videos**: Automatically converts MKV → MP4  
  - **Audio**: Automatically converts M4A, WMA → FLAC
- **Documents**: Automatically converts Office formats (DOC, DOCX, PPT,
PPTX, XLS, XLSX) → PDF, and processes text-based formats (HTML,
Markdown, CSV) as plain text
- Refactors format definitions by removing the complex `formatSupport`
struct and replacing it with simple arrays for Gemini-supported and
convertible formats
- Removes the `validateFormat` function that only provided error
messages, replacing it with direct format checking and conversion logic
- Provides clear error messages for truly unsupported formats that
cannot be converted
- Ensures consistent behavior across all media processing paths (both
inline data and File API)

Author: pinglin