[Bug][http-client-js] ser/deser for unixTimestamp type has issues

### Describe the bug

Regarding the spector test cases - [encode/datetime for unixTimestamp](https://github.com/microsoft/typespec/blob/main/packages/http-specs/specs/encode/datetime/main.tsp#L54-L77), when converting unixTimestamp to number there exist issues in ser/deser and that's why we have to [skip these cases](https://github.com/microsoft/typespec/blob/fba94cc316febb05deb166e3bbd8a41bcbd2cadd/packages/http-client-js/test/e2e/http/encode/datetime/main.test.ts#L64-L79).

### Reproduction

NA

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.