Simplify models

[dgageot]

No description provided.

[docker-agent]

Assessment: 🟢 APPROVE

This PR simplifies the modelsdev package by removing unused fields and streamlining the code structure. The refactoring is clean and maintains the same functionality.

Changes reviewed:

• ✅ Removed unused fields from Provider and Model structs
• ✅ Removed unused CachedAt and UpdatedAt timestamps
• ✅ Converted NewStore to use sync.OnceValues directly (cleaner singleton pattern)
• ✅ Changed GetProvider to getProvider (unexported, internal use only)
• ✅ Removed unused ModelSupportsReasoning function
• ✅ Simplified Bedrock region prefix handling (inlined helper function)
• ✅ Updated test fixtures to match simplified structs

Concurrency safety: The mutex usage in GetDatabase and getProvider is correct. The cached database is immutable after loading, making concurrent reads safe.

No bugs found in the changed code. The simplification improves code maintainability without introducing issues.

[rumpl]

Can we stop simplifying? It's going in circles at this point

[rumpl]

Do we really have to remove the fields? It's a public package and external might need it?