Pin the problem+json error/negotiation contract

[turegjorup]

Closes the last untested corner of the error contract: what the API does when a client asks for application/problem+json.

Changes

Two new ErrorContractTest cases (verified against the running 4.3 API):

• Auth error as problem+json — GET /api/v2/events (unauthenticated) with Accept: application/problem+json → 401, content-type: application/problem+json, RFC 7807 fields (title, detail, status, type). The 401 short-circuits before resource negotiation, so it is served in the requested media type.
• problem+json not acceptable for resource reads — GET /api/v2/events/{id} with Accept: application/problem+json → 406 Not Acceptable (the resources only produce ld+json), and the 406 body is itself a problem+json error.

Why

The regenerated 4.3 spec documents application/problem+json/application/json media types on error responses, but nothing tested whether the API actually serves them. This pins the real behaviour: problem+json is available for errors that precede resource negotiation (401), but is not a resource representation — asking for it on a resource yields 406, not the resource and not a 404. Consumers must read resources as ld+json. Tests only.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.99%. Comparing base (191e1ed) to head (7fa98e3).
⚠️ Report is 11 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #52   +/-   ##
==========================================
  Coverage      72.99%   72.99%           
  Complexity       232      232           
==========================================
  Files             29       29           
  Lines            648      648           
==========================================
  Hits             473      473           
  Misses           175      175           

Flag	Coverage Δ
unittests	72.99% <ø> (ø)

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