[IshwarKanse]

Problem

main's test job has been red since #934 was merged (d17ce87):

--- FAIL: TestSearchResponseEmptyTracesPreserved
--- FAIL: TestTraceByIDResponseEmptyResourceSpansPreserved
    unsupported content type: ""

This is a semantic conflict between two PRs that each passed CI on their own:

• fix: preserve empty traces/resourceSpans arrays in RBAC-filtered trace responses #934 added the two tests above. They build &http.Response{Header: http.Header{}} with no Content-Type and relied on unmarshal() falling back to JSON.
• fix: trace query RBAC: do not try to unmarshal unsupported content types #937 (merged Sept 29) made unmarshal()/marshal() return unsupported content type for anything other than protobuf or JSON.
• fix: preserve empty traces/resourceSpans arrays in RBAC-filtered trace responses #934's last CI run was on refs/pull/934/merge = d467c43 into 08aad9a, a base from before fix: trace query RBAC: do not try to unmarshal unsupported content types #937, and it was merged on Oct 13 without a re-run.

Fix

Set Content-Type: application/json in both tests, the same default the makeResponse helper in this file already uses. Test-only change. The assertions still go through the JSON unmarshaler, so the empty-array round-trip they protect is still covered, and #937's stricter content-type handling is unchanged.

Verification

• go test -race -short on all unit packages (e2e excluded, as in make test-unit) passes.
• go vet and gofmt are clean.
• Before the change, the two tests fail on main (0b8e429) with the error above.

The test-e2e failure on the d17ce87 push (TestMetricsReadAndWrite/OIDC_redirect_protection) is unrelated: #934 doesn't touch the metrics path, and test-e2e is green on the next main push (0b8e429).

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