Remove dependency on obsolete 'codecov' gem

[mvz]

What this does

This removes use of the obsolete 'codecov' gem.

The codecov gem is obsolete and coverage information should be uploaded using the codecov-cli tool.

This repository already uses the Codecov GitHub Action, which uses the codecov-cli tool. This only requires coverage info to be formatted using the CoberturaFormatter. The simplecov configuration already took care of this. The present change only removes the codecov dependency and use of its formatter.

See https://github.com/codecov/codecov-ruby?tab=readme-ov-file for the deprecation notice of the codecov gem, and https://docs.codecov.com/docs/deprecated-uploader-migration-guide#ruby-uploader for the setup needed for the modern codecov-cli reporter.

This change was triggered by me noticing that codecov 0.6.0 requires ruby < 4, so the existing setup installed version 0.2.1 on my system.

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

Required for new features

• I opened an issue before writing code and received maintainer approval
• Linked issue: #___

PRs for new features or enhancements without a prior approved issue will be closed.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.13%. Comparing base (c72846b) to head (f956ece).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #625   +/-   ##
=======================================
  Coverage   80.13%   80.13%           
=======================================
  Files         113      113           
  Lines        5095     5095           
  Branches     1307     1307           
=======================================
  Hits         4083     4083           
  Misses       1012     1012           

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