Fix image conversion state handling across formats

[emmanuel-ferdman]

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Why are these changes needed?

This PR fixes critical issues in image format conversion:

• Undefined image variable - Ensured proper initialization for URL/local/bytes formats
• Method argument mismatch - Removed unexpected parameter from convert_image_to_base64()
• Null buffer references - Standardized BytesIO initialization across all formats
• Missing format validation - Added explicit error for unsupported image types

Related issue number (if applicable)

Checks

• [x] I've run format.sh to lint the changes in this PR.
• [ ] I've included any doc changes needed.
• [x] I've made sure the relevant tests are passing (if applicable).

[fuckgitb]

您好，我最近正在休假中，无法亲自回复您的邮件。我将在假期结束后，尽快给您回复。

[emmanuel-ferdman]

@infwinston @CodingWithTim could you please take a look into this small PR when you get the chance? thanks 🙌