Fix compatibility issue with URL attachments wrong number of arguments

[DustinFisher]

• Fix ArgumentError: wrong number of arguments (given 2, expected 0..1) that occurs when Marcel processes URI objects
• Use filename-based MIME detection for URLs instead of passing URI objects to Marcel
• Add test that reproduces the issue and validates the fix
• Maintains backward compatibility and improves performance by avoiding network calls

What this does

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
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

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

Related issues

Fixes #247

[crmne]

Thank you for this!

I suggested a readability improvement.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.72%. Comparing base (f33e5d8) to head (066f622).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #250   +/-   ##
=======================================
  Coverage   89.72%   89.72%           
=======================================
  Files          75       75           
  Lines        2812     2812           
  Branches      555      556    +1     
=======================================
  Hits         2523     2523           
  Misses        289      289           

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