Minor improvements to disagg test coverage and suppression hygiene:

• test_config06.py: Remove the skip_for_hook("disagg", ...) from test_storage_tier_config_in_asc. The test now runs under disagg where it is also valid (ASC does not support storage_tier=cold, but the hook rewrites it to a layered URI so the test is compatible).
• test_config_disagg01.py: New test file verifying DSC-specific session.create configuration: disaggregated=(storage_tier=cold). Disagg tests are automatically skipped in hook.
• test_sweep01.py: Remove skip_for_hook("disagg", ...) from the class and add the test to hook_disagg.fail with a FIXME reference to WT-16982 (disagg doesn't yet sweep layered dhandles; tracked there for long-term fix).