Simplify rulebased router: remove redundant types and score aggregation

[dgageot]

• Remove route struct; store []Provider directly since route.model duplicated provider.ID()
• Simplify selectProvider: use Size=1 instead of fetching 10 hits and aggregating scores by route (top-1 hit is always the best match)
• Extract parseRouteIndex helper for doc ID parsing
• Consolidate index cleanup on error paths with a single defer
• Remove unused 'route' numeric field from Bleve index
• Compute filterOutMaxTokens once instead of per providerFactory call
• Trim verbose comments; update tests to match

Assisted-By: docker-agent

[docker-agent]

Review Summary

Assessment: 🟢 APPROVE

This refactoring successfully simplifies the rule-based router implementation by:

• Removing the redundant route struct and storing []Provider directly
• Optimizing search to fetch only the top-1 hit instead of 10 (since Bleve returns hits sorted by score)
• Extracting parseRouteIndex helper for cleaner code organization
• Consolidating error-path cleanup with a defer pattern
• Removing the unused 'route' numeric field from the Bleve index
• Computing filterOutMaxTokens once at the start

The code changes are well-structured and maintain correctness:

• The defer cleanup pattern correctly uses a local cleanupErr variable to conditionally close the index on error paths
• The parseRouteIndex function properly validates route indices with defensive checks
• Test coverage has been updated to match the simplified implementation

No bugs or issues found in the changed code. The refactoring improves code clarity without introducing behavioral changes.