perf(people): bind Employment-separation verdicts to JSON response media type #387

Finding

The current Employment-separation buyer-path harness parses response.body with the strict JSON boundary and validates status/body semantics, but it does not validate the response media type before accepting a governed success/conflict verdict.

Canonical People HTTP truth on current #64 emits Content-Type: application/json for _send_json(...), including /v1/employment-separations. On current #317, parseJson(response) ignores response.headers, so a proxy/service returning the same governed-looking body under text/plain, a missing Content-Type, or an ambiguous duplicate case-variant Content-Type can still contribute a passing check and a clean employment_separation_unexpected_response Rate. That is a commercial-evidence false-GREEN: the transport no longer proves the published JSON representation that the harness claims to measure.

Required repair

Test first, then minimally:

• add a response-contract boundary that accepts one own Content-Type field whose media type is application/json (case-insensitive, optional parameters allowed) and rejects missing/non-string/non-JSON/ambiguous case-variant duplicates;
• invoke that boundary before parsing the response body in the buyer workload so first-commit, replay, rejection, and contention all fail closed on the same transport contract;
• retain the existing duplicate-aware raw JSON body parsing, closed success/error envelopes, operational UUID/timestamp/replay checks, pinned k6 runtime and outcome/latency cardinality contracts;
• do not add gateway/provider assumptions or weaken the p(95) <= 20 ms acceptance boundary.

Keep open through protected/integration acceptance; branch-local repair is not a close condition.