fix(plpgsql-deparser): handle already hydrated expressions in hydrateExpression

[pyramation]

Summary

Fixes a bug where hydratePlpgsqlAst would crash with TypeError: query.indexOf is not a function when called on an already-hydrated AST. This occurred because hydrateExpression expected query to always be a string, but when processing an already-hydrated AST, the query property is a HydratedExprQuery object.

The fix adds a check at the beginning of hydrateExpression to detect if the query is already hydrated (using the existing isHydratedExpr function) and return it unchanged.

Use case: This is needed for schema transformation pipelines where the same AST might be processed multiple times, such as in constructive-db's schema name transformer.

Review & Testing Checklist for Human

• Verify the isHydratedExpr function correctly identifies all hydrated expression kinds (raw, sql-stmt, sql-expr, assign)
• Confirm the type signature change from query: string to query: string | HydratedExprQuery doesn't cause TypeScript issues downstream
• Run full test suite: pnpm test in packages/plpgsql-deparser

Test plan: After merging and publishing, update constructive-db to use the new version and verify the schema transformation pipeline works without the query.indexOf error.

Notes

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

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