Summary

• Canonicalize PostgreSQL datetime type aliases before comparing db_type overrides in MatchesColumn, so timestamptz, timestamp with time zone, and pg_catalog.timestamptz are treated as equivalent
• Add unit tests for timestamptz alias matching and an end-to-end testdata fixture verifying both spellings generate the same overridden Go type

Fixes #2914

Test plan

• go test ./internal/codegen/golang/opts/... -run TestMatchesColumnTimestamptzAliases
• go test ./internal/endtoend/... -run 'TestReplay/base/overrides_timestamptz_alias'