fix: Set Content-Type header for JSON responses in serializeHTTPHandler

[slim-bean]

The serializeHTTPHandler was missing the Content-Type header for JSON responses, causing clients to receive "text/plain; charset=utf-8" instead of the expected "application/json; charset=UTF-8".

This bug was introduced in commit d0c11a6 (Dec 2024) when Parquet support was added. The Parquet code path correctly sets the Content-Type header, but the JSON code path (the default) was left without any Content-Type header.

The issue became visible in the last 4 weeks as the v2 engine router changes (commits 72cb649 and d721511) made more query_range requests flow through this code path.

This fix ensures JSON responses from /loki/api/v1/query_range and other query endpoints have the correct Content-Type header set, matching the behavior in other parts of the codebase (see codec.go lines 686, 1304).

Fixes: https://github.com/grafana/loki/issues/[issue-number] (cherry picked from commit f27ccf6)

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