Add Atlas Cloud provider support

[binyangzhu000-sudo]

Summary

• add Atlas Cloud as an OpenAI-compatible chat provider
• register atlascloud configuration options and default API base
• add live-verified Atlas Cloud text model metadata and provider capability tests

Validation

• ruby -c lib/ruby_llm/providers/atlascloud.rb
• ruby -c lib/ruby_llm/providers/atlascloud/capabilities.rb
• ruby -c spec/ruby_llm/providers/atlascloud/capabilities_spec.rb
• parsed lib/ruby_llm/models.json and verified Atlas Cloud model entries

No README changes.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.18%. Comparing base (b667fbe) to head (d860f74).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/ruby_llm/providers/atlascloud.rb	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #841      +/-   ##
==========================================
- Coverage   83.28%   83.18%   -0.11%     
==========================================
  Files         172      175       +3     
  Lines        8282     8497     +215     
  Branches     1347     1373      +26     
==========================================
+ Hits         6898     7068     +170     
- Misses        888      929      +41     
- Partials      496      500       +4     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[binyangzhu000-sudo]

Thanks. I pushed follow-up commit d860f749a24872ae154dede857146e70de866b63 addressing the failing RuboCop path offense.

What changed:

• Renamed spec/ruby_llm/providers/atlascloud/capabilities_spec.rb to spec/ruby_llm/providers/atlas_cloud/capabilities_spec.rb, matching RuboCop's expected path for RubyLLM::Providers::AtlasCloud::Capabilities.
• Kept the test content unchanged.

Validation:

• ruby -c spec/ruby_llm/providers/atlas_cloud/capabilities_spec.rb
• ruby -c lib/ruby_llm/providers/atlascloud.rb
• ruby -c lib/ruby_llm/providers/atlascloud/capabilities.rb
• git diff --cached --check
• git diff --check origin/main...HEAD

Notes:

• I did not run paid Atlas live API calls.
• The local host Ruby is 2.6.10 while this gem requires Ruby >= 3.1.3, and Docker is not running locally, so the full bundled RuboCop/RSpec suite is left to the freshly queued PR CI.