Refactor: Extract helper functions to reduce sync/async duplication (Phase 1)

[jayendra13]

Summary

Phase 1 of refactoring to eliminate duplicated code between sync and async Zarr reading paths.

• Extract apply_cf_time_conversion() helper to cf_time.rs for CF time conversion logic
• Add helper functions in zarr_reader.rs to consolidate repeated patterns:
  • create_empty_result_stream() - Empty result stream for unmatched filters
  • build_projected_schema() - Schema building with coordinate filtering
  • coord_values_to_refs() - CoordValues to CoordValuesRef conversion
  • calculate_effective_sizes() - Filter-based size calculation
  • extract_filtered_coords() - Coordinate value extraction with ranges
  • calculate_query_coord_sizes() - Mixed-dimensionality size calculation
  • create_result_batch() - Final batch creation with empty projection handling
  • apply_limit_to_arrays() - Limit application to result arrays

Impact

• Net change: -2 lines (283 additions, 285 deletions)
• Improved maintainability: sync and async paths now share common logic
• No functional changes - all tests pass

Test plan

• cargo test - All tests pass
• cargo clippy - No warnings
• cargo fmt - Properly formatted

Part of #5