Add hazard for new NOT NULL columns requiring backfill

[catatsuy]

Description

Add a new hazard for schema diffs that add a new column as NOT NULL without a DEFAULT.
When this pattern is detected, the generated plan includes a clear warning and guidance for a safer online approach (add the column nullable, backfill data, then set NOT NULL using the existing online NOT NULL workflow).

Motivation

ADD COLUMN ... NOT NULL without a DEFAULT is commonly unsafe for non-empty tables and can lead to failed migrations or unexpected blocking.
pg-schema-diff already supports online NOT NULL changes for existing columns, but for new columns the tool cannot infer how to backfill data. This change helps operators notice the risk early and nudges them toward a staged migration.

Testing

• Added/updated tests to cover:

  • Adding a new NOT NULL column without a DEFAULT emits the new hazard and guidance.
  • Adding a new NOT NULL column with a constant DEFAULT does not emit the hazard (or follows the intended behavior).

• Ran the test suite locally (go test ./...).

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