Report Concurrency Slots Free When the Build Finishes

report.generate and report.synchronous now release an organization's concurrency slot as soon as the underlying report build reaches a terminal state (complete or failed), rather than when a client happens to observe that terminal state. Abandoning a request — timing out, giving up on retries, or never polling at all — no longer holds a slot for the rest of its lifetime.
The per-organization limit of 3 concurrent report operations, shared across both endpoints, is unchanged.