server: settle live payment before LV2V EOS

[fastagents]

Summary

Settle the final live V2V payment interval before the gateway publishes EOS and
cancels the stream context.

Problem

The gateway currently handles EOSReader before the normal per-segment payment
call. Meanwhile, the orchestrator accounts payment units from elapsed time when
it receives the next segment.

A delayed final partial segment followed by EOS can therefore cause the
orchestrator to account an interval that the gateway never funds. If the
remaining balance is insufficient, the stream may close with an
insufficient-balance error during normal EOS handling.

Changes

• Process the outstanding payment interval synchronously before publishing
  EOS.
• Serialize synchronous and asynchronous payment processing to prevent
  overlapping settlement.
• Add regression coverage confirming final settlement completes before context
  cancellation.

Testing

• go test ./server -run '^TestLivePaymentProcessor' -count=1
• go test -race ./server -run '^TestLivePaymentProcessor' -count=1
• go test ./server -count=1