Strip ambient profile from host-scoped Databricks auth

[AarushiShah-db]

Issue:

• ucode still keeps ambient DATABRICKS_CONFIG_PROFILE in build_databricks_cli_env().
• Before ucode resolves a profile, has_valid_databricks_auth(workspace, profile=None) can call databricks auth token --host with
  DATABRICKS_CONFIG_PROFILE=other-workspace still set.
• I reproduced that locally with a fake databricks binary: has_valid=True and the subprocess saw DATABRICKS_CONFIG_PROFILE=other-workspace.

Change:

• Updated src/ucode/databricks.py so host-scoped Databricks CLI calls strip ambient DATABRICKS_CONFIG_PROFILE when no explicit profile is being passed.
• Threaded the explicit profile through has_valid_databricks_auth, run_databricks_login, and get_databricks_token.
• Added regression tests in tests/test_databricks.py for the stale profile shadowing case.

Validation:

• uv run pytest tests/test_databricks.py -q → 60 passed
• uv run ruff check src/ucode/databricks.py tests/test_databricks.py → passed
• uv run pytest -q → 546 passed, 25 skipped
• uv run ruff check . → passed

[rohita5l]

@AarushiShah-db Changes in agents/databricks.py looks good but confused about changes in agents/codex.py

[rohita5l]

@AarushiShah-db approved the PR. but will need to resolve the merge conflicts