google-cloud-firestore: Accept values of generic `Mapping` type (e.g. Python 3.15 `frozendict`) in addition to `dict` as field values when creating / replacing / merging a document in the Firestore database

### Determine this is the right repository

- [x] I determined this is the correct repository in which to report this feature request.

### Summary of the feature request

I'd like to be able to use other dictionary types, custom ones and `frozendict` (Python 3.15+), when adding or updating documents in Firestore without having to create new `dict` objects by wrapping my instance (`dict(my_mapping)`).

Looking at the code, the input value doesn't need to be _mutable_ at all (as `dict` is), and it only needs to support `.items()`, so it really can be any valid `collections.abc.Mapping`:

https://github.com/googleapis/google-cloud-python/blob/3bbcbdf1a62dd10ab96f72e5c8198cffe9174ac7/packages/google-cloud-firestore/google/cloud/firestore_v1/_helpers.py#L234-L245

That means the code here

https://github.com/googleapis/google-cloud-python/blob/3bbcbdf1a62dd10ab96f72e5c8198cffe9174ac7/packages/google-cloud-firestore/google/cloud/firestore_v1/_helpers.py#L224-L227

could be simply checking

```python
if isinstance(value, collections.abc.Mapping):
...
```

### Desired code experience

```python
from collections import UserDict

class UpperCaseDict(UserDict):
def __setitem__(self, key, item):
key = key.upper()
super().__setitem__(key, item)

record = {"my_dict": UpperCaseDict({"one": 1, "two": 2})}

db = firestore.client(...)
db.collection("cities").add(record) # no errors
```

### Expected results

No errors for valid mapping types:

https://github.com/googleapis/google-cloud-python/blob/3bbcbdf1a62dd10ab96f72e5c8198cffe9174ac7/packages/google-cloud-firestore/google/cloud/firestore_v1/_helpers.py#L229-L231

### API client name and version

google-cloud-firestore v2.30.0

### Use case

Uploading my custom mapping instances directly without conversion.

### Additional context

A few valid mapping types currently cause an exception:

```python
from collections import UserDict, defaultdict
from collections.abc import Mapping
from types import MappingProxyType

class UpperCaseDict(UserDict):
def __setitem__(self, key, item):
key = key.upper()
super().__setitem__(key, item)

d = {"one": 1, "two": 2}
dd = defaultdict(int, d)
proxy = MappingProxyType(d)
fd = frozendict(one=1, two=2) # Python 3.15+
uppercase = UpperCaseDict({"one": 1, "two": 2})

print(isinstance(d, Mapping)) # True
print(isinstance(d, dict)) # True

print(isinstance(dd, Mapping)) # True
print(isinstance(dd, dict)) # True

print(isinstance(proxy, Mapping)) # True
print(isinstance(proxy, dict)) # False

print(isinstance(fd, Mapping)) # True
print(isinstance(fd, dict)) # False

print(isinstance(uppercase, Mapping)) # True
print(isinstance(uppercase, dict)) # False
```