fix(firestore): remove usage of typing_extensions (#17357)

Fixes #17244 🦕

This PR addresses the `google-cloud-firestore` portion of #17244
("Remove usage of typing_extensions in google-cloud-python") by
replacing the only `typing_extensions` import in this package with the
stdlib `typing` equivalents.

## Summary

`google/cloud/firestore_v1/async_transaction.py` was the sole module in
`google-cloud-firestore` importing from `typing_extensions`. All three
names it imports (`Concatenate`, `ParamSpec`, `TypeVar`) are available
in the stdlib `typing` module since Python 3.10, and `setup.py` declares
`python_requires=">=3.10"`, so the import can be moved to the stdlib
without any compatibility loss.

## Changes


`packages/google-cloud-firestore/google/cloud/firestore_v1/async_transaction.py`:
- Add `Concatenate`, `ParamSpec`, `TypeVar` to the existing `from typing
import (...)` block (kept alphabetical).
- Remove the `from typing_extensions import Concatenate, ParamSpec,
TypeVar` line.

## Why this works

| Symbol | Stdlib `typing` availability |
|---|---|
| `Concatenate` | Python 3.10+ |
| `ParamSpec` | Python 3.10+ |
| `TypeVar` | Always available |

## Verification

In a clean venv with only `google-cloud-firestore` installed (no
`typing-extensions`), `from google.cloud.firestore_v1 import
async_transaction` now succeeds — confirming the dependency on
`typing-extensions` is fully removed.

## Scope note

Other packages in this monorepo also use `typing_extensions`
(`google-api-core`, `google-cloud-bigtable`, `google-cloud-spanner`,
`bigframes`). Each has its own `python_requires` and set of imported
symbols (notably `Self`, which requires Python 3.11+ in the stdlib), so
this PR is scoped to `google-cloud-firestore` only. Let me know if you'd
like me to follow up with PRs for any of the others — happy to.

Author: shokkunrf

packages/google-cloud-firestore/google/cloud/firestore_v1/async_transaction.py

@@ -22,13 +22,15 @@
     AsyncGenerator,
     Awaitable,
     Callable,
+    Concatenate,
     Generic,
     Optional,
+    ParamSpec,
+    TypeVar,
 )
 
 from google.api_core import exceptions, gapic_v1
 from google.api_core import retry_async as retries
-from typing_extensions import Concatenate, ParamSpec, TypeVar
 
 from google.cloud.firestore_v1 import _helpers, async_batch
 from google.cloud.firestore_v1.async_document import AsyncDocumentReference