Fix missing models.dev date handling for created_at metadata

[afurm]

What this does

Fixes an edge case in Models.models_dev_model_to_info where created_at was built from a blank date when both release_date and last_updated were missing.

Before this fix, it could build " 00:00:00 UTC", which later parsed to the current date, causing incorrect and non-deterministic created_at values in model metadata.

This PR:

• Sets created_at only when a non-blank date is present.
• Keeps created_at as nil when both source dates are missing.
• Adds a regression test covering this nil-date case.

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 81.37%. Comparing base (97c0546) to head (9daceb1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #652   +/-   ##
=======================================
  Coverage   81.36%   81.37%           
=======================================
  Files         116      116           
  Lines        5425     5427    +2     
  Branches     1426     1427    +1     
=======================================
+ Hits         4414     4416    +2     
  Misses       1011     1011           

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