fix(query-engine): normalize topk labels in binary expressions

[milindsrivastava1997]

Summary

Fixes #631. PromQL topk(...) binary arms now use the original non-__name__ labels for intermediate vector matching. Standalone Topk output formatting remains unchanged.

The fix covers instant and range paths, including topk + plain, topk + topk, scalar arithmetic, empty joins, and all supported arithmetic operators.

Validation

• Full query_engine_rust test suite
• cargo fmt
• cargo check
• cargo clippy
• Pre-commit hooks

All checks pass.