refactor(query-engine): expose PromQL execution errors

[milindsrivastava1997]

Summary

• make the public PromQL instant/range methods return Result<Option<_>, QueryExecutionError>
• propagate native execution errors through regular and binary query paths, while retaining Ok(None) for capability misses and no local data
• return native execution failures locally as HTTP 500 responses without forwarding them to Prometheus

Validation

• cargo test --workspace
• cargo clippy -p query_engine_rust --all-targets -- -D warnings
• pre-commit hooks (format, check, clippy, test)