bfloat16s (at least) throw error when given a format specifiier

```
>>> import jax
>>> jax.__version__
'0.4.21'
>>> from jax.dtypes import bfloat16
>>> f"{bfloat16(1)]}"
'1'
>>> f"{bfloat16(1):.2f}"
Traceback (most recent call last):
File "", line 1, in
ValueError: Unknown format code 'f' for object of type 'str'
```

This is a little surprising to me and, in a larger context, the error message isn't very suggestive about what or where the error is.

Start by reproducing the reported bfloat16 formatting examples through the jax.dtypes.bfloat16 entry point, then locate the formatting implementation and its existing tests. Add coverage for the .2f case and verify that the resulting behavior or error clearly identifies the bfloat16 formatting problem.

Written by the indexing model from the issue text.