CONSTRUCT_QUERY_RE skipped comments with a bare \#[^\n]*, whose trailing *
backtracks. For '# CONSTRUCT ...\nSELECT ...' the engine gave back everything
after the '#', so the CONSTRUCT inside the comment satisfied the query-form
keyword and a SELECT/ASK was reported as a CONSTRUCT.
All four SPARQL backends delegate to this regex, so such a query took the
CONSTRUCT branch of execute_sparql, which sends Accept: text/turtle and parses
the body as Turtle — failing with a misleading 'Failed to parse CONSTRUCT
response as Turtle'.
Require a comment to reach a line terminator. Both LF and CR are accepted
because the SPARQL grammar ends a comment at either; matching only LF would
regress CR-terminated comments into false negatives.
Add regression tests covering both directions across all four backends.>
