fix(schema): clear enumValues when enum validator is removed with enum(false) - #16631

Summary

Calling enum(false) (or enum(undefined)) on a String or Number path removes the enum validator but leaves the old values in schemaType.enumValues. This causes two problems:

• toJSONSchema() still emits enum: [...] for that path, so the generated schema rejects values Mongoose itself now accepts.
• enum() appends to enumValues, so re-enabling the enum brings the old values back. After enum(false) and enum('c'), the validator still allows 'a' and 'b'.

This PR resets enumValues in the branch of SchemaString.prototype.enum() and SchemaNumber.prototype.enum() that removes the validator. Calling enum() with values still appends, as before.

Examples

const schema = new Schema({ status: { type: String, enum: ['a', 'b'] } });
schema.path('status').enum(false);

schema.path('status').enumValues; // ['a', 'b'] before, [] now
schema.toJSONSchema().properties.status; // still had enum: ['a', 'b', null] before

Added a test in test/schema.validation.test.js that fails on master and passes with this change.