feat: Complete schema diffing with map support

Completes the schema diffing implementation with comprehensive map support:
- Map key and value type tracking and comparison
- Map value nullability change detection (loosened/tightened)
- Struct keys and values within maps
- Complex deeply nested scenarios mixing structs, arrays, and maps
- Comprehensive edge case testing

Test additions:
- test_map_value_struct_field_changes: Tests field changes within map values
- test_map_nullability_loosened: Tests map value nullability relaxation (safe)
- test_map_nullability_tightened: Tests map value nullability restriction (breaking)
- test_map_with_struct_key: Tests maps with struct keys
- test_cursed_deeply_nested_complex_types: Tests extremely nested combinations

This is the final part (5/5) of the schema diffing feature implementation.

Related to #1346

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Co-Authored-By: Claude <[email protected]>

Author: murali-db