Report task duration on Java outcome events and workflow nodes #483

Java exposes only raw timestamps, so every consumer recomputes elapsed time by hand.

• sdks/java/src/main/java/org/byteveda/taskito/events/OutcomeEvent.java — no duration field
• sdks/java/src/main/java/org/byteveda/taskito/workflows/NodeSnapshot.java — raw startedAt/completedAt longs only

Add a durationMs() accessor on both (and on the middleware task context if it has the timestamps), matching what the other SDKs report.