fix: bound stream accounting finalization

[Marketen]

Issue

PR #2 delays finish_reason and [DONE] until trailing usage and accounting finish. Its five-second timer only cancels the provider read, while proof persistence and RecordSuccess run with context.WithoutCancel. If metering is unreachable, its three five-second attempts can therefore hold a normal streaming response for about 15 seconds; Tinfoil proof persistence can extend that to about 30 seconds.

Fix

Start one absolute five-second finalization deadline when the completion event is observed, and apply the remaining budget to the cancellation-detached proof and metering context. This preserves accounting after a client disconnect, but proof/metering retries can no longer outlive the response finalization budget. Bare-EOF streams receive the same bounded accounting context.

Example

Before: provider finishes at t=0; metering is down; [DONE] may be held until roughly t=15s (t=30s for Tinfoil).

After: provider finishes at t=0; the shared deadline expires at t=5s; outstanding accounting calls are canceled and the gateway emits the retained completion.

Validation

• go test ./...
• go vet ./...
• git diff --check
• Extended the stream accounting test to assert that metering receives the bounded finalization deadline.