sql/postgres: treat serial as equivalent to integer + nextval default

[wokalski]

Summary

A column declared as serial in PostgreSQL is semantically identical to an integer column with a DEFAULT nextval('<table>_<column>_seq') clause. The PostgreSQL inspector already normalizes the latter to a SerialType (see columnDefault in inspect.go), but the HCL parser does not — so a state-based diff between the two declarative forms reports a spurious ChangeType + ChangeDefault change.

Example that previously produced a diff but no longer does:

// BEFORE
table "test" {
  schema = schema.test
  column "id" {
    null = false
    type = serial
  }
}

// AFTER
table "test" {
  schema = schema.test
  column "id" {
    null = false
    type = integer
    default = sql("nextval('test.test_id_seq'::regclass)")
  }
}

Approach

Two commits:

1. d9d7853d — reproduction: adds three subtest cases to TestDiff_TableDiff covering serial ↔ integer + nextval, bigserial ↔ bigint + nextval, and the reversed direction. Without the fix these tests fail with ChangeType | ChangeDefault.

2. 6404929a — fix: implements sqlx.Normalizer on the postgres diff driver. The Normalize method rewrites any IntegerType column whose Default is a RawExpr matching nextval('<table>_<column>_seq'[::regclass]) into the equivalent SerialType with the default cleared, mirroring the existing inspection-time normalization. Both from and to tables are normalized before diffing, so both declarative forms collapse to the same representation. CockroachDB's crdbDiff still overrides Normalize with its own logic, so it is unaffected.

Also adds an HCL-level end-to-end test TestDiff_SerialVsIntegerNextval exercising the user's exact scenario through the HCL parser.

Test plan

• go test ./sql/postgres/... — passes including the new tests
• go test ./... — passes everywhere except atlasexec (pre-existing failure: tests require an atlas CLI binary in $PATH, unrelated)
• Manual smoke against a real PostgreSQL DB (sequence must exist; this PR only touches the diff path, not validation)

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