Fix PostgreSQL column DEFAULT expression analysis

[rebelice]

Summary

• Analyze column DEFAULT expressions with PostgreSQL column-default semantics instead of standalone expression semantics.
• Return DEFAULT analysis and coercion errors for CREATE TABLE and ALTER TABLE paths, with rollback for failed ADD COLUMN/default analysis.
• Match PostgreSQL error precedence and SQLSTATEs for column references, subqueries, aggregates, grouping, window/SRF defaults, and identity/generated default changes.

Test Plan

• go test ./pg/catalog -count=1
• git diff --check