Python timestamp hashes disagree with Scala at subsecond precision

### Feature Summary

Python tuple hashing converts timestamp keys to whole epoch seconds, while Scala delegates to Java `Timestamp.hashCode`, which uses epoch milliseconds. Equal timestamp keys can therefore choose different workers in a hash shuffle when Scala and Python producers feed the same downstream operator.

Expected behavior:

Python tuple hashes match Java tuple hashes at timestamp millisecond precision.

Reproduction evidence:

Use three receivers and the timestamp `1970-01-01T00:00:00.500Z`.

Before: Python hash 31 selects worker 1, Scala hash 531 selects worker 0
After: both hashes are 531 and both select worker 0

Negative, zero, and positive millisecond boundaries are covered.

Version and commit evidence:

Current `main` at 70c21145887920528d7d5540e3fb790b43e8b759.

**Commit Hash (Optional)**

`70c21145887920528d7d5540e3fb790b43e8b759`

### Proposed Solution or Design

Expected behavior:

### Affected Area

Workflow Engine (Amber)