test(gepa): add comprehensive ReAct detection and reconstruction tests

Detection tests (3):
- test_single_react_module_detection: top-level ReAct module
- test_multi_react_workflow_detection: mixed ReAct + ChainOfThought (bug fix validation)
- test_nested_react_orchestrator_worker_detection: orchestrator with 2 workers as tools

Reconstruction tests (3):
- test_build_program_single_react: single ReAct module
- test_build_program_multi_react_workflow: mixed workflow with ReAct + non-ReAct
- test_build_program_orchestrator_with_workers: complex nested structure

Helper functions (12):
- setup_spy_for_base_program: captures base_program from gepa.optimize
- simple_metric_for_detection/reconstruction: test metrics
- create_gepa_optimizer_for_detection: creates optimizer
- assert_react_module_detected/updated: validates ReAct modules
- assert_regular_module_detected/updated: validates non-ReAct modules
- mock_optimized_react_module: mocks optimized candidate
- create_*_program: 3 reusable program builders

Validates:
- Full path preservation (bug fix)
- All 4 ReAct components (react, extract, tools, arg_desc)
- Non-ReAct module handling
- Deepcopy verification (original unchanged)
- Both detection and reconstruction phases

Author: Ju-usc