Unix time the day before a leap second seems incorrect

### Description
```python
t0 = Time('1978-12-31T23:59:59', format='isot', scale='utc')
t0.to_value('unix')
```
returns 283996798.0000231 instead of 283996799

in general it seems that all unix time stamp the day before a leap second are one second off.

### Expected behavior

unix epoch of '1978-12-31T23:59:59' should be 283996799

### How to Reproduce
```python
from astropy.time import Time
t0 = Time('1978-12-31T23:59:59', format='isot', scale='utc')
t0.to_value('unix')
```

### Versions

Linux-5.10.102.1-microsoft-standard-WSL2-x86_64-with-glibc2.31
Python 3.12.6 (main, Sep 10 2024, 00:05:17) [GCC 9.4.0]
astropy 6.1.4
Numpy 2.1.1
pyerfa 2.0.1.4
Scipy not installed
Matplotlib not installed

Start by running the supplied reproduction and trace Time.to_value('unix') through the UTC conversion path. Use the reported 1978-12-31T23:59:59 case as the regression target; done means it returns 283996799 rather than the shown off-by-one value.

Written by the indexing model from the issue text.