[fs/azure] Set auth.type=Custom for token delegation

[affo]

Purpose

Linked issue: close #2946

Token delegation does not seem to work when the client does not configure any entry of the Azure FS configuration:

1. FlussConnection filters client config:
   FileSystem.initialize(
   Configuration.fromMap(extractPrefix(conf.toMap(), "client.fs.")), null);
   1. extractPrefix filters to client.fs.* keys but does not strip the prefix — so keys remain as client.fs.* in the resulting config.
   2. AzureFileSystemPlugin.getHadoopConfiguration looks for keys with prefix "fs.azure." — but the client config has no such keys (only client.fs.*). The Hadoop config receives no Azure credentials.
   3. setCredentialProvider (AzureFileSystemPlugin.java:64-84): since fs.azure.account.key is absent from the Hadoop config, it falls back to delegation tokens and calls AzureDelegationTokenReceiver.updateHadoopConfig(hadoopConfig).
   4. updateHadoopConfig (AzureDelegationTokenReceiver.java:41-71):
      - Sets fs.azure.account.oauth.provider.type = DynamicTemporaryAzureCredentialsProvider
      - Sets additionInfos entries (just fs.azure.account.oauth2.client.endpoint)
      - DOES NOT set fs.azure.account.auth.type
   5. ABFS driver (AzureBlobFileSystem.initialize → AzureBlobFileSystemStore. → initializeClient): since fs.azure.account.auth.type is not set, defaults to SharedKey auth → calls SimpleKeyProvider.getStorageAccountKey → key not found → "Failure to
      initialize configuration".

Brief change log

Simply hadoopConfig.set("fs.azure.account.auth.type", "Custom"); in case no account key is set (presumably, client side).

Tests

• Fix existing UT test that was skipping an exception thrown on FS init to actually check the exception is not due to misconfig
• add check for "fs.azure.account.auth.type", "Custom" to existing UTs
• add IT test to simulate the token delegation path (the previous IT test only checked the "server path" by setting the complete config)

API and Format

NA

Documentation

NA