fix(cpp-httplib-server): handle missing optional JSON properties

[Dharshika-11]

Fixes #23991

This fixes a crash in cpp-httplib-server generator when optional JSON properties
are missing in the request payload.

Previously, NLOHMANN_DEFINE_TYPE_INTRUSIVE was used which does not safely handle
missing fields.

Now replaced with custom to_json/from_json using:

• j.at() for required fields
• j.contains() check for optional fields

---

Summary by cubic

Prevents crashes in the cpp-httplib-server generator when request JSON omits optional fields by switching to explicit to_json/from_json handling. Generated servers now safely ignore missing optional properties.

• Bug Fixes
  • Replaced NLOHMANN_DEFINE_TYPE_INTRUSIVE with friend to_json/from_json in model headers.
  • Required fields use j.at(...).get_to(...).
  • Optional fields use j.contains(...) and null check before get_to(...).

^{Written for commit 3eeffcd. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/27406589837/job/81020413323?pr=24010

please follow step 3 in the PR checklist to update the samples so as to address the build failure.