feat: Add nested struct support to schema diffing

Extends schema diffing to handle nested structures:
- Recursive field collection through struct hierarchies
- Ancestor filtering to report only LCA changes
- Type change classification for struct containers
- Support for arbitrarily deep nesting

Arrays and maps support coming in follow-up PRs.

This is part 3/5 of the schema diffing feature implementation.

Related to #1346

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

Author: murali-db