…rays); avoid lazy type for enum unions

Context:
When a DTO property used a union like Enum | null (or Enum[] | null) the plugin sometimes:

• emitted a lazy type: () => FooEnum (unnecessary for enums; can cause circular refs), or
• missed enum: FooEnum entirely for optional+nullable cases (e.g. ?: Status | null).

Root cause:

• createTypePropertyAssignments didn’t special-case enum unions, so it fell back to a lazy type.
• createEnumPropertyAssignment could return early when the checker reported a union containing null/undefined or when array unwrapping wasn’t detected up front, dropping the enum.

What changed:

1. Handle enum unions in createTypePropertyAssignments:

   • For unions with exactly one non-nullish member: * If that member (or its array element) is an enum (including auto-generated enum unions), do not emit type. * Only add nullable: true when null is part of the union.
   • This avoids circular resolvers and lets enum metadata be provided solely by createEnumPropertyAssignment.

2. Rewrite createEnumPropertyAssignment to be union- and optional-safe:

   • Unwrap null/`undefined` from unions before enum detection.
   • Treat array unwrapping as optional (don’t bail if not detected initially).
   • Resolve auto-generated enum unions (string literal unions) to a referenceable type.
   • Preserve and/or re-detect isArray after resolving enum-like types.
   • Skip enum members (e.g., FooEnum.ONE) which aren’t referenceable as enum types.

Results:

• Enum | null → { enum: FooEnum, nullable: true }
• Enum[] | null → { enum: FooEnum, isArray: true, nullable: true }
• ?: Enum | null → { required: false, enum: FooEnum, nullable: true }
• Non-enum unions (e.g., string | null) still emit type + nullable (as before).
• undefined continues to affect required (based on ?), not nullable — unchanged.

Tests:

• Fixture test/plugin/fixtures/nullable.dto.ts asserts correct transpiled metadata.
• Previously failing spec “should support & understand nullable type unions” now passes.

Affects:

• lib/plugin/visitors/model-class.visitor.ts
• test/plugin/fixtures/nullable.dto.ts

Fixes: #3519

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information