withRequestID honored any inbound X-Request-Id verbatim, and that value is
echoed on the response, embedded in the error envelope, and persisted into
audit_logs.request_id. An unvalidated caller-supplied id is therefore an
audit-integrity vector: an arbitrarily long value bloats the audit row, and a
stray control byte (CR/LF) could smuggle a forged entry into a log sink.
Accept an inbound id only when it is well-formed — non-empty, at most 64
bytes, and restricted to a log-safe charset ([A-Za-z0-9._-]) — otherwise mint
a fresh server id. A rejected request loses its inbound trace link, which is
strictly better than storing attacker-controlled text in the audit trail.
