fix: use json.Marshal in WriteJSONResponse to avoid trailing newline

[lpcox]

Problem

Commit f9f875b refactored two sites in internal/proxy/handler.go to use httputil.WriteJSONResponse instead of manual w.Write calls. However, WriteJSONResponse used json.NewEncoder(w).Encode(body), which always appends a trailing newline. This caused 8 proxy tests to fail:

• TestWriteEmptyResponse (6 subtests)
• TestHandleWithDIFC_LabelResponseError_CoarseBlocked
• TestHandleWithDIFC_NoFineGrainedLabels_CoarseBlocked

All failures showed the same pattern: expected "[]" but got "[]\n".

Fix

Switch WriteJSONResponse from json.NewEncoder(w).Encode(body) to json.Marshal(body) + w.Write(data), which produces clean JSON without a trailing newline. This is the standard approach for HTTP JSON responses.

Verification

make agent-finished passes — all unit and integration tests green.

[Copilot]

Pull request overview

This PR fixes proxy test failures by changing httputil.WriteJSONResponse to emit JSON without the trailing newline produced by json.Encoder.Encode.

Changes:

• Replace json.NewEncoder(w).Encode(body) with json.Marshal(body) followed by w.Write(data) to avoid a trailing newline.
• Add early-return handling for marshal failures.

Show a summary per file

File	Description
internal/httputil/httputil.go	Updates JSON response writing to avoid trailing newline in HTTP responses.

Copilot's findings

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• Files reviewed: 1/1 changed files
• Comments generated: 1

[Copilot]

json.Marshal is done after WriteHeader(statusCode). If marshaling fails, this returns after headers/status have already been sent, leaving an empty body with the original status code (often 200) and no indication of the server error. Consider marshaling first, and on error writing an explicit 500 (or at least the correct error status) before sending headers; also consider handling/logging the w.Write error.

See below for a potential fix:

	data, err := json.Marshal(body)
	if err != nil {
		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
		return
	}

	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(statusCode)
	if _, err := w.Write(data); err != nil {
		return
	}