Investigate Content-Encoding zstd on Azure SDK starting Python 3.14

Starting Python 3.14, Python starts to send through Accept-Encoding that it accepts "zstd", for instance:
```
Accept-Encoding: gzip, deflate, zstd
```

We need to investigate what happens if Azure picks up on that, and starts to return an encoded response with that format, with a header like that in response:
```
Content-Encoding: zstd
```

We do have code that check encoding with an exhaustive list, that may be at risk in 3.14. For instance:
https://github.com/Azure/azure-sdk-for-python/blob/483968503410f4b2a28e9948221d8f5d7ac20ec7/sdk/core/azure-core/azure/core/pipeline/transport/_aiohttp.py#L457