test(pg-delta): add PG15+ roundtrip coverage for NULLS NOT DISTINCT unique indexes

[Copilot]

PostgreSQL 15+ unique indexes can use NULLS NOT DISTINCT, which changes duplicate-NULL semantics. pg-delta already models and serializes this attribute, but it lacked end-to-end integration coverage to prove the clause survives extract → diff → apply → re-read roundtrips and directional toggles.

• Added PG15+ roundtrip coverage

  • Covers creating a unique index with NULLS NOT DISTINCT
  • Verifies full declarative/diff roundtrip fidelity for the clause

• Added directional toggle coverage

  • Plain unique index → same index with NULLS NOT DISTINCT
  • NULLS NOT DISTINCT unique index → plain unique index

• Made clause loss fail loudly

  • Added inline snapshots for generated migration statements so regressions are visible at the SQL level, not just via catalog equality

• Scope

  • Test-only change in existing index integration coverage
  • No product code changes were needed

Example covered by the new tests:

CREATE UNIQUE INDEX idx_accounts_email
  ON test_schema.accounts USING btree (email)
  NULLS NOT DISTINCT;

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Latest commit: 59cd705

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commit: 59cd705