Missing type validation in joins and set operations

[dtsong]

Problem

data_diff/queries/ast_classes.py:486,556 — TODO comments indicate missing schema type compatibility checks for joins and set operations.

Impact

Type mismatches in joins/unions could produce incorrect results silently.

Suggested Fix

Add type compatibility validation when combining table schemas.

References

• File: data_diff/queries/ast_classes.py, lines 486, 556

[dtsong]

Set operation type validation was fixed in commit 5c2a6ba. The remaining Join TODO has been clarified with a proper comment.