[BUG] [typescript] incorrect `Date` serialization for `format: date`

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The current implementation uses non-UTC methods to serialize dates using format: date.

https://github.com/OpenAPITools/openapi-generator/blob/f834519aebf8a565b1be768e67b1bd0aa8242b25/modules/openapi-generator/src/main/resources/typescript/model/ObjectSerializer.mustache#L112-L119

This approach causes incompatibility with deserialization, which expects a UTC date.

Here's a screenshot that demonstrates the issue

openapi-generator version

Tested 7.1.0 and master. Not a regression.

Generation Details

openapi-generator-cli generate --generator-name="typescript" --additional-properties="enumPropertyNaming=original,modelPropertyNaming=original"

Steps to reproduce

Generate any spec file containing a date field with format: date.

Related issues/PRs

I couldn't find any issue or PR mentioning this.

Suggest a fix

Replace current serialization implementation with a solution that is used in other TS generators

https://github.com/OpenAPITools/openapi-generator/blob/f834519aebf8a565b1be768e67b1bd0aa8242b25/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache#L136-L138

I'll create a PR with proposed changes later today.

Start with modules/openapi-generator/src/main/resources/typescript/model/ObjectSerializer.mustache at the referenced serialization code, then compare the analogous logic in typescript-fetch/apis.mustache. Generate a TypeScript client from a spec containing a date field with format: date and verify that serialization produces a UTC-compatible value that deserialization can read.