Fix inconsistency in Partial String Index with 'second' resolution
- Closes #14826
- Tests added: test_partial_string_indexes. Comprehensive testing of partial string indexes.

## Whatsnew entry
Fix inconsistency in Partial String Index with 'second' resolution. See #14826. Now if the timestamp and the index both have resolution `second`, timestamp is considered as an exact match try and not a slice. Therefore, for `Series`, scalar will be returned, for `DataFrame` `KeyError` raised.