BUG: Series constructor loses date unit of Timestamps

[WillAyd]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

>>> pd.Timestamp("2024-01-01 00:00:00").unit
's'
>>> pd.Series([pd.Timestamp("2024-01-01 00:00:00")]).dt.unit
'ns'

### Issue Description

I would expect the Series to maintain the unit of 's'

### Expected Behavior

I would expect the Series to maintain the unit of 's'

### Installed Versions

'3.0.0.dev0+681.g434fda08cf'

[WillAyd]

@jbrockmendel in case you have any insights into this

[jbrockmendel]

Addressed by #55901