chore(deps): update dependency typing-extensions to v4.15.0 (googleapis#746)

This PR contains the following updates:

| Package | Change | Age | Confidence |
|---|---|---|---|
| [typing-extensions](https://redirect.github.com/python/typing_extensions) ([changelog](https://redirect.github.com/python/typing_extensions/blob/main/CHANGELOG.md)) | `==4.14.1` -> `==4.15.0` | [![age](https://developer.mend.io/api/mc/badges/age/pypi/typing-extensions/4.15.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/pypi/typing-extensions/4.14.1/4.15.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) |

---

### Release Notes

<details>
<summary>python/typing_extensions (typing-extensions)</summary>

### [`v4.15.0`](https://redirect.github.com/python/typing_extensions/blob/HEAD/CHANGELOG.md#Release-4150-August-25-2025)

[Compare Source](https://redirect.github.com/python/typing_extensions/compare/4.14.1...4.15.0)

No user-facing changes since 4.15.0rc1.

</details>

---

### Configuration

📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

---

This PR was generated by [Mend Renovate](https://mend.io/renovate/). View the [repository job log](https://developer.mend.io/github/googleapis/python-spanner-sqlalchemy).
<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiI0MS44Mi43IiwidXBkYXRlZEluVmVyIjoiNDEuODIuNyIsInRhcmdldEJyYW5jaCI6Im1haW4iLCJsYWJlbHMiOltdfQ==-->

Author: renovate-bot

requirements.txt

@@ -527,9 +527,9 @@ tomli==2.2.1 \
     --hash=sha256:ece47d672db52ac607a3d9599a9d48dcb2f2f735c6c2d1f34130085bb12b112a \
     --hash=sha256:f4039b9cbc3048b2416cc57ab3bda989a6fcf9b36cf8937f01a6e731b64f80d7
     # via -r requirements.in
-typing-extensions==4.14.1 \
-    --hash=sha256:38b39f4aeeab64884ce9f74c94263ef78f3c22467c8724005483154c26648d36 \
-    --hash=sha256:d1e1e3b58374dc93031d6eda2420a48ea44a36c2b4766a4fdeb3710755731d76
+typing-extensions==4.15.0 \
+    --hash=sha256:0cea48d173cc12fa28ecabc3b837ea3cf6f38c6d1136f85cbaaf598984861466 \
+    --hash=sha256:f0fa19c6845758ab08074a0cfa8b7aecb71c999ca73d62883bc25cc018c4e548
     # via
     #   alembic
     #   opentelemetry-api