Returns the SAME report GET /most-used-keys serves, rendered as an RFC 4180 CSV
attachment, for the same limit the JSON endpoint accepts.

One computation, two representations. The figures are produced once, by the same
service call the JSON endpoint makes, and then written.

What the file states about itself. A #-led preamble carries the report name, the
scope the caller read under, the population (the top limit keys of the whole
inventory in scope, ranked by usageCount descending — and a note that
totalUsageCount covers the WHOLE inventory, not only the rows listed), the
completeness (BOUNDED: this file is the top N the request asked for, a complete
answer to THAT question and not a page of a longer one), and then EVERY member the JSON
response publishes under its own JSON name — including usageWindow and
usageWindowKey, so the total never appears without the span it is measured over, the
two served band boundaries, and generatedAt, which is the report's own instant.

The header row IS KeyUsageInfo's member list in declaration order. An unlimited key
renders an EMPTY usagePercentage cell, never a zero — a percentage of an absent limit
does not exist. The file terminates with # EXPORT COMPLETE; its ABSENCE means the
stream broke before the end.

Why produces names application/json as well as text/csv: the success
representation is CSV, but every refusal is an RFC 9457 application/problem+json body,
which a text/csv-only mapping would make unselectable.

Authorization: the report's own scope — admin.tenant.analytics.read.