IdempotencyStore keyed a replayed submission on the raw Idempotency-Key
header alone. Two different callers who send the same header value
shared one cache slot: the second caller received the first caller's
cached reference instead of running its own submission.
Program.cs now composes the key as "{SubjectId}:{idemKey}" in the
Submit helper, so the cache is scoped per caller. Add a test that
proves a caller cannot replay another caller's idempotency key and
receive their cached result.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
