feat(schema): include ObjectId regex pattern in toJSONSchema() output

[AbdelrahmanHafez]

Closes #16334

schema.toJSONSchema() rendered ObjectId paths (including _id) as { type: 'string' }, which puts no constraint on the value. This adds pattern: '^[A-Fa-f0-9]{24}$' so the portable schema can validate ObjectId strings when fed into ajv, zod, etc.

The pattern is only added to the plain JSON Schema representation. The useBsonType: true output is unchanged, since bsonType: 'objectId' already validates ObjectIds and that's the representation used for MongoDB $jsonSchema collection validators.

new Schema({ author: { type: Schema.Types.ObjectId } }).toJSONSchema();
// {
//   type: 'object',
//   required: ['_id'],
//   properties: {
//     author: { type: ['string', 'null'], pattern: '^[A-Fa-f0-9]{24}$' },
//     _id: { type: 'string', pattern: '^[A-Fa-f0-9]{24}$' }
//   }
// }

[Copilot]

Pull request overview

Adds ObjectId string validation constraints to Mongoose’s portable schema.toJSONSchema() output by including a hex ObjectId regex pattern, while keeping the MongoDB $jsonSchema-oriented { useBsonType: true } output unchanged.

Changes:

• Update SchemaObjectId#toJSONSchema() to include pattern: '^[A-Fa-f0-9]{24}$' when not using useBsonType.
• Update existing AJV-based tests to use valid 24-hex _id values now that the schema is stricter.
• Add a focused regression test covering ObjectId paths (including _id) for both portable JSON Schema and useBsonType modes (gh-16334).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lib/schema/objectId.js	Adds ObjectId pattern to portable JSON Schema output; skips it for useBsonType where bsonType: 'objectId' is used.
test/schema.test.js	Updates expected schema snapshots and AJV validations; adds regression coverage for ObjectId pattern behavior.
test/schema.union.test.js	Updates union schema expectations and AJV validations to use valid ObjectId-like _id strings.