[TEST ONLY] Add group role assumption support

[mihaimitrea-db]

Summary

Adds group role assumption support to the Python SDK. This draft PR mirrors the implementation from the Universe source branch so the OSS repository isolated integration workflow can validate it against real Databricks endpoints.

Test-only OSS PR: do not merge. The implementation must merge through Universe.

Why

Universe does not run tests/integration/. The standalone SDK repository has the privileged isolated test workflow needed to exercise group-role workload identity federation against a UC account and workspace.

What changed

Interface changes

• Adds group_id configuration through constructor arguments, DATABRICKS_GROUP_ID, and configuration profiles.
• Exposes group_id on WorkspaceClient and AccountClient.

Behavioral changes

• Sends assume_group during supported Databricks OAuth token exchanges.
• Prevents unsupported authentication strategies from silently ignoring a configured group.
• Keeps account and unified-host validation server-side and surfaces server errors.

Internal changes

• Separates external-browser token caches by group.
• Adds unit and integration coverage for group propagation, refresh/re-exchange, strategy selection, configuration precedence, and WIF permission isolation.

How is this tested?

• Universe runtests --quiet: 94/94 selected tests passed.
• Credentials-provider suite: 112/112 passed.
• Related config, OAuth, OIDC, notebook OAuth, and model-serving suites passed.
• This PR is being opened specifically to run the OSS isolated integration workflow.