Fix estimation procedure extraction for clustering tasks

[kartikmandar]

Summary

• Add TaskType.CLUSTERING to the estimation procedure extraction block in _create_task_from_xml()
• Handle empty estimation_procedure_id and estimation_procedure_type (some clustering tasks have None in XML)
• Handle empty target_name for unsupervised tasks
• Add tests for clustering task estimation procedure extraction

Test plan

• [x] Test clustering task 126033 (has complete estimation procedure data)
• [x] Test clustering task 146714 (has empty estimation procedure fields)
• [x] Verify classification/regression tasks still work correctly

Fixes #1522