Fix inlineData and text not handled correctly for Gemini

[bubiche]

What this does

• Gemini/Vertex AI can response with text and inlineData together, e.g. {"candidates": [{"content": {"role": "model", "parts": [{"text": "Here is your image! "}, {"inlineData": {"mimeType": "image/png", "data": ... }}]}. The current code doesn't handle inlineData because extract_text_parts would just get the text and be done with it.
  => Fix it by making parse_content the only code path, it'll filter out non-user-visible stuff like thought and functionCall and build content correctly.
• I think there's also a bug inRubyLLM::Providers::Gemini::Media's build_response_content, Content.new(text:, attachments:) expects attachment sources (paths/IO/URLs), not RubyLLM::Attachment instances
  => Fix it to construct Content correctly too.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

AI-generated code

• I used AI tools to help write this code
• I have reviewed and understand all generated code (required if above is checked)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes