Tag values always decoded as immutable

### Things to check first

- [x] I have searched the existing issues and didn't find my bug already reported there

- [x] I have checked that my bug is still present in the latest release

### cbor2 version

6.0.1

### Python version

3.14.4

### What happened?

When decoding tagged maps, they are always returned as `frozendict`, regardless of the value of the `immutable` flag.

I assume this is a bug? Maybe related to #295?

### How can we reproduce the bug?

```
>>> from cbor2 import loads
>>> print(loads(bytes.fromhex("CAA10000")))
CBORTag(10, frozendict({0: 0}))
>>> print(loads(bytes.fromhex("A10000")))
{0: 0}
```