`AZURE_LOG_LEVEL` should support `VERBOSE` and/or not fail if level wasn't recognized

1. Python SDK should support the entire range of `AZURE_LOG_LEVEL` values that other SDKs (Java, C++) do, i.e. it should support `VERBOSE`. `VERBOSE` is the same as `DEBUG` that the SDK currently supports, there just needs to be an alias.

Here is more info: https://github.com/Azure/azure-sdk-for-cpp/tree/48530b7cb3d18660359f278047ecb72119296a7e/sdk/core/azure-core#sdk-log-messages

2. If the value of environment variable `AZURE_LOG_LEVEL` is not recognized, Python SDK should not be failing.
Currently:
```
/usr/bin/az login ...

ERROR: Cannot convert verbose to log level, valid values are: CRITICAL, ERROR, WARNING, INFO, DEBUG

...

File "/opt/az/lib/python3.13/site-packages/azure/core/settings.py", line 147, in convert_logging
raise ValueError("Cannot convert {} to log level, valid values are: {}".format(value, ", ".join(_levels)))
ValueError: Cannot convert verbose to log level, valid values are: CRITICAL, ERROR, WARNING, INFO, DEBUG
To check existing issues, please visit: https://github.com/Azure/azure-cli/issues

```