fix(rag): honor ctx cancellation in Initialize/Query fan-in collectors

[aheritier]

Summary

pkg/rag/manager.go:219-225 (Initialize) and :352-360 (Query) receive from resultsChan without selecting on ctx.Done(); a strategy that hangs (ignoring ctx) blocks the caller forever. Buffered channels already prevent goroutine leaks; only the caller hang remains.

Fix

select on ctx.Done() in both collect loops.

Acceptance

• A hanging strategy no longer blocks the caller past ctx cancellation; test.