Optimize Provider Name Extraction with Allowed Provider Scope

[wilsonccccc]

What Changed

• Simplified provider name extraction logic by leveraging database-level intersection
• Removed redundant adapter factory checks (user's provider keys are the authoritative source)
• Enhanced support for custom providers not in the adapter factory

Key Improvements

• Performance: Eliminated redundant set intersection operations by using pre-filtered allowed_provider_names
• Security: allowed_provider_names already represents the intersection of user's provider keys and API key scope
• Maintainability: Single function handles both optimized and comprehensive provider lookup
• Custom Providers: Now works with any custom provider configured by users