Fix: Resolve type checker error and add mypy_cache to gitignore

- Line 446: Replace 'key=transcript_lengths.get' with 'key=lambda tid: transcript_lengths[tid]'
  to resolve 'Argument key has incompatible type' type checker error
- Add .mypy_cache to .gitignore to prevent type checking cache files from being staged

Author: kunal-10-cloud

.gitignore

@@ -1,6 +1,7 @@
 .idea
 .vscode
 __pycache__
+.mypy_cache
 *.pyc
 dist
 .venv/

malariagen_data/anoph/genome_features.py

@@ -443,7 +443,7 @@ def canonical_transcript(
             raise ValueError(f"Gene '{gene}' has no transcripts with exons")
 
         # Find canonical (maximum length)
-        canonical = max(transcript_lengths, key=transcript_lengths.get)
+        canonical = max(transcript_lengths, key=lambda tid: transcript_lengths[tid])
         canonical_length = transcript_lengths[canonical]
         debug(f"Canonical transcript: {canonical} with {canonical_length} bp")
         return canonical