fix(schema): avoid throwing error on array of unions

[vkarpov15]

Re: #15718

Summary

Array of unions currently throws an error:

/home/v/Desktop/MongoDB/mongoose/lib/schema/union.js:23
    this.schemaTypes = options.of.map(obj => options.parentSchema.interpretAsType(key, obj, schemaOptions));
                                                                  ^

TypeError: Cannot read properties of undefined (reading 'interpretAsType')
    at /home/v/Desktop/MongoDB/mongoose/lib/schema/union.js:23:67
    at Array.map (<anonymous>)
    at new Union (/home/v/Desktop/MongoDB/mongoose/lib/schema/union.js:23:35)
    at new SchemaArray (/home/v/Desktop/MongoDB/mongoose/lib/schema/array.js:95:21)

This would have been fixed separately by the refactor in #15700

Examples

[Copilot]

Pull Request Overview

This PR fixes support for arrays of union types (gh-15718). Previously, defining unions inside arrays would fail because the parentSchema property wasn't being set correctly for union types within arrays.

• Sets parentSchema on union type definitions when they appear inside arrays
• Adds test coverage for arrays of unions with mixed types (Number and Date)

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/schema.js	Sets parentSchema property on union types within array definitions
test/schema.union.test.js	Adds test case verifying that arrays of unions work correctly with different value types

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