Fix course instructor serialization with Koha REST context

[nugged]

Addresses #44.

Problem

CourseReservesController::_get_paged_results serialized result sets directly with:

$results->to_api()

That bypasses the request context supplied by Koha::REST::Plugin::Objects. In particular, Koha::Patron->to_api requires the authenticated user parameter.

This explains the data-dependent failure reported in #44:

• no instructors: the empty result set serializes as [];
• one or more instructors: patron serialization is invoked without user, raising Koha::Exceptions::MissingParameter and returning HTTP 500.

Change

Serialize paged course-reserve result sets through Koha's controller helper:

$c->objects->to_api($results)

The helper supplies the standard public, embed, strings, and user context. This fixes non-empty /instructors responses without introducing plugin-local serialization policy.

A separate commit records the fix under an Unreleased changelog section so release maintenance can adjust it independently.

Commits

1. Use Koha REST serializer for course reserve results
   • functional one-line fix with the failure mechanism documented in the commit body;
2. Document the course instructor serialization fix
   • changelog-only commit.

Test plan

1. Request /api/v1/contrib/kohasuomi/instructors with no instructors and confirm HTTP 200 with [].
2. Add a patron as instructor to an enabled course.
3. Request the endpoint again and confirm HTTP 200 with the instructor's patron representation.
4. Confirm pagination headers remain present.
5. Smoke-test /courses, /departments, and /coursereserves, which share _get_paged_results.

Scope note

This PR does not alter the patron_id selected by /coursereserves. The controller currently sources it from course_instructors.borrowernumber; a concrete mismatching response is needed to determine whether #44 compares that internal patron identifier with userid/card number or exposes a separate join/cardinality defect.

Verification

The branch is exactly two commits ahead of main. The functional diff is one changed line; the other changed file is CHANGELOG.md.

Runtime tests were not run here because this repository has no test harness and the execution environment does not contain a Koha installation.