feat: Add 62 new unit tests for match_regions_to_genes, transcript, candidate processing, and parsers

[TianYuan-Liu]

• match_regions_to_genes integration tests (9 tests): empty regions/genes
  handling, sorted region processing, max_gene_length effects, multiple
  chromosomes, large gaps between genes, gene level reporting with merging,
  and region order preservation.

• Transcript advanced tests (7 tests): set_length interactions with
  calculate_size, exons extending beyond boundaries, renumber with many
  exons for both strands, overlapping exons robustness, single exon
  transcripts, and clone preserving exon numbers.

• Process candidates edge cases (7 tests): single candidate at each
  reporting level (exon/transcript/gene), multiple genes at gene level,
  same gene different transcripts merging, mixed areas at transcript
  level, and all-below-threshold handling.

• Config validation tests (8 tests): empty/comma-only rules parsing,
  partial valid rules, invalid tag handling, multiple set_distance_kb
  calls, all-zero and large max_lookback values, default rules order.

• TSS boundary condition tests (5 tests): region at exact TSS position,
  very large distances, negative strand at end, zero length TSS zone,
  large region spanning all zones.

• TTS boundary condition tests (5 tests): region at exact TTS position,
  very large distances, negative strand at start, zero TTS zone, large
  region spanning TTS and downstream.

• BED reader edge cases (6 tests): single line files, exact chunk size,
  chunk larger than file, mixed comments, browser/track lines, Windows
  line endings.

• GTF parser edge cases (6 tests): single exon genes, many exons (20),
  unsorted exons, mixed strands on same chromosome, genes without exons,
  duplicate exons.

• Rules priority tests (5 tests): FirstExon beats Promoter, TSS beats
  FirstExon, custom rules with Downstream first, same priority ties,
  pctg_region tiebreaker.

• Output line format validation (5 tests): field count with/without
  metadata, field order verification, percentage rounding, hundred
  percent formatting.

Unit test count increased from 282 to 344 tests.