WI-003: #18 ProblemDetails has custom MarshalJSON but no UnmarshalJSON — problem responses can't round-trip

[rluders]

Work item WI-003 of work package WP-001 (GitHub issues).

`ProblemDetails.MarshalJSON` flattens `Extensions` to the top level per RFC 9457 (`type`/`title`/`status`/`detail`/`instance` plus arbitrary extension keys, all siblings). There's no corresponding `UnmarshalJSON`, so decoding a problem response back into a `ProblemDetails` loses every extension member — `json.Unmarshal` only populates the struct's declared fields and silently drops the rest, since `Extensions` isn't itself a tagged field that captures leftover keys.

This bites API clients and tests that need to decode a problem response and inspect an extension (e.g. a `messageKey` i18n hint, or a `correlationId`) — there's no way to get it back out via the type this library provides.

Requested: a matching `UnmarshalJSON` that reads the known fields and stuffs everything else into `Extensions`, so `ProblemDetails` round-trips through JSON.

Implemented by the Weave Autoloop, attempt 1, execution exe_4feb2775d52c.

Changed files:

• problem_details.go
• problem_details_test.go

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Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.35%. Comparing base (9b55b5e) to head (57a0e40).

Files with missing lines	Patch %	Lines
problem_details.go	91.66%	1 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   83.60%   84.35%   +0.75%     
==========================================
  Files          14       14              
  Lines         500      524      +24     
==========================================
+ Hits          418      442      +24     
  Misses         63       63              
  Partials       19       19              

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[rluders]

Weave review: approved (execution exe_ab89e8a56b69)

• Reviewer by agent go-reviewer: approved — The change meets the work item. I read the code and tests and checked the step report. I did not re-run the tests myself.

Acceptance criteria

1. A matching UnmarshalJSON reads the known fields.

   • Implemented at problem_details.go:52-84.
   • The known fields (type, title, status, detail, instance) are decoded at problem_details.go:53-62.
   • It uses a pointer receiver, and it assigns *p only after both decodes succeed (problem_details.go:72-82), so a failed decode leaves the target unchanged.
   • Tests: TestProblemDetailsJSONRoundTrip (problem_details_test.go:24) checks the known fields. TestProblemDetailsUnmarshalJSONInvalid (problem_details_test.go:69) checks that a bad status type returns an error.

2. Every other top-level member goes into Extensions.

   • Implemented at problem_details.go:64-81. The known keys are deleted from the member map (problem_details.go:68-70), and the rest become Extensions.
   • Test: TestProblemDetailsJSONRoundTrip (problem_details_test.go:33-36) uses messageKey and correlationId, the two examples from the work item.

3. ProblemDetails round-trips through JSON.

   • The round-trip test compares the original and decoded values with reflect.DeepEqual (problem_details_test.go:49).
   • TestProblemDetailsUnmarshalJSONNoExtensions (problem_details_test.go:54) shows that a payload with no extensions leaves Extensions nil, even if the target had stale extensions before.

4. Public API is documented, and the change is minimal.

   • UnmarshalJSON has a doc comment (problem_details.go:50-51).
   • Only problem_details.go and its test file changed.

5. go vet ./... and go test -count=1 ./... both exit 0, according to report.json.

Minor notes (not blocking)

• MarshalJSON skips an extension keyed "extensions" (problem_details.go:41), but UnmarshalJSON would keep a wire member with that name. That only affects unusual payloads and doe…
• Person: approved — Operator review (Orkestra dogfood operator; this is NOT a GitHub review): diff read. UnmarshalJSON mirrors MarshalJSON: known RFC 9457 members typed, every other top-level member into Extensions, nil Extensions when none, wrong types rejected. Tests cover round trip, no extensions, invalid input. CI 8/8 green. Note for the maintainer: numeric extension values decode as float64 (encoding/json default). Ready for a human merge decision on GitHub; Orkestra will not merge.