Test sliding window observer behavior

## Goal

Validate sliding window observer works correctly across kernel lifecycle scenarios.

## Test Coverage

**Unit tests:**

- Timestamp tracking logic
- Window expiration calculation
- Cleanup trigger conditions

**Integration tests:**

- Normal kernel lifecycle (start → run → destroy)
- Rapid kernel creation/destruction
- Observer behavior during metric collection gaps
- No metric leaks or premature cleanups

## Verification

- Verify metrics are cleaned up after configured window
- Verify no cleanup failures during kernel destruction
- Monitor production metrics after deployment

## Dependencies

Requires completed implementation (BA-3521 subtasks 2 and 3).

JIRA Issue: BA-3525