HandleResponseBody unconditionally called all latency metric helpers at end-of-stream. Those helpers enforce timestamp ordering and log errors when timestamps are zero — causing deterministic CI failures in tests that exercise parsing or response-size accumulation without a full request lifecycle (no timing fields set).

Changes

• pkg/epp/handlers/response.go: Wrap each latency metric call in the presence check it requires:
  • RecordNormalizedTimePerOutputToken / RecordRequestLatencies: only when RequestReceivedTimestamp and ResponseCompleteTimestamp are non-zero
  • RecordRequestTTFT: only when RequestReceivedTimestamp and FirstTokenTimestamp are non-zero
  • RecordRequestTPOT: only when all three timestamps are non-zero
  • RecordResponseSizes: always recorded (no timestamps required)

if endOfStream {
    if !reqCtx.RequestReceivedTimestamp.IsZero() && !reqCtx.ResponseCompleteTimestamp.IsZero() {
        metrics.RecordNormalizedTimePerOutputToken(...)
        metrics.RecordRequestLatencies(...)
    }
    metrics.RecordResponseSizes(...)
    if !reqCtx.RequestReceivedTimestamp.IsZero() && !reqCtx.FirstTokenTimestamp.IsZero() {
        metrics.RecordRequestTTFT(...)
    }
    if !reqCtx.RequestReceivedTimestamp.IsZero() && !reqCtx.FirstTokenTimestamp.IsZero() && !reqCtx.ResponseCompleteTimestamp.IsZero() {
        metrics.RecordRequestTPOT(...)
    }
}

• pkg/epp/handlers/response_test.go: Add TestHandleResponseBodyZeroTimestamps — verifies that end-of-stream with zero timestamps emits no latency metrics and still records response size.

Production behaviour is unchanged: in a real request all timestamps are set before HandleResponseBody is called, so the guards are never triggered.