feat(plpgsql-deparser): enable heterogeneous deparse for AST-based transformations

[pyramation]

feat(plpgsql-deparser): enable heterogeneous deparse for AST-based transformations

Summary

This PR enables AST-based transformations (e.g., schema renaming) to be properly reflected in deparsed PL/pgSQL function bodies. Previously, modifications to AST nodes in hydrated expressions were ignored because dehydrateQuery() returned the original string fields instead of deparsing the modified AST.

Key changes:

• Added deparseExprNode() helper that wraps expression AST nodes in a SELECT statement, deparses, and strips the prefix
• Updated dehydrateQuery() for sql-expr kind to always prefer deparsing the AST node
• Updated dehydrateQuery() for assign kind to prefer deparsing targetExpr/valueExpr AST nodes when available
• Updated hydrate-demo.test.ts to demonstrate proper AST-based modifications (instead of string field modifications)
• Added tests for schema renaming across all hydrated expression kinds

⚠️ Breaking change: String-based modifications to query.target, query.value, and query.original are now ignored if AST nodes exist. Users should modify AST nodes directly for transformations.

Review & Testing Checklist for Human

• Verify breaking change is acceptable: Previously, modifying string fields like query.original would work. Now AST modifications are preferred. Confirm this behavioral change is intentional and acceptable for downstream users.
• Check dead code: The normalizeForComparison() function is defined but appears unused in the final implementation - should it be removed?
• Test with real PL/pgSQL functions: The snapshot shows formatting differences (e.g., ::type → CAST(... AS type), multi-line OR conditions). Verify these don't break downstream consumers.
• Verify type reference limitation: Type references in DECLARE sections (e.g., v_user "schema".users) use string typname fields, not AST nodes, so they won't be transformed. Confirm this limitation is documented/acceptable.

Recommended test plan:

1. Run pnpm test in packages/plpgsql-deparser to verify all 26 tests pass
2. Test schema renaming on a real PL/pgSQL function with various expression types
3. Verify the deparsed output is valid SQL that can be executed

Notes

• Link to Devin run: https://app.devin.ai/sessions/f8178d7485484832b5507d277bbf2919
• Requested by: Dan Lynch (@pyramation)

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