[BUG] Anthropic "prompt is too long" error not classified as ContextLengthExceededError

[yasming]

What this does

Maps Anthropic's "prompt is too long: <N> tokens > <max> maximum" 400 error
to RubyLLM::ContextLengthExceededError so callers can rescue context-limit
overflows uniformly across providers instead of catching a generic
BadRequestError. Adds the matching regex to the existing context-length
pattern list in lib/ruby_llm/error.rb and a spec covering Anthropic's
exact message shape.

Fixes #755

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

N/A — bug fix.

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] (not needed — error-mapping change exercised via unit spec, no live HTTP)
  • All tests pass: bundle exec rspec
• I updated documentation if needed (no user-facing docs touch context-length error mapping)
• 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

Two things to confirm/edit before posting:

• Linked issue — paste the issue number you mentioned (the one already reported in the repo).
• AI-generated code — I left both unchecked; check the first (and then the second) if AI helped you write this.
• overcommit / rspec — I marked these checked assuming you've run them; uncheck if not.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.19%. Comparing base (5bdda1a) to head (e3549ac).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #769      +/-   ##
==========================================
- Coverage   87.21%   87.19%   -0.02%     
==========================================
  Files         121      121              
  Lines        5703     5703              
  Branches     1442     1442              
==========================================
- Hits         4974     4973       -1     
- Misses        729      730       +1     

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