Preserve debounce upstream failure without outstanding demand

[Hokila]

Summary

Preserve upstream failures in debounce when there is no outstanding downstream demand.

Root Cause

When the upstream threw while DebounceStateMachine was in waitingForDemand with no buffered element, the state machine transitioned to .finished. This discarded the original error, causing the next call to next() to return nil instead of throwing.

Changes

• Preserve the upstream error using .upstreamFailure(error:).
• Continue cancelling the debounce task and clock continuation.
• Add public API regression tests covering:
  • Upstream failure without producing any element.
  • Failure after an element was produced while downstream had no demand.
  • Non-zero debounce intervals.
  • Preservation of the original error type and value.
  • No additional elements or repeated completion after failure.

Validation

• swift test --filter TestDebounce
• swift test
• swift-format lint --recursive Sources Tests

All checks passed on macOS arm64.

Fixes #450