Reorganize graph tests and add device-side launch support

[Andy-Jost]

Summary

This PR reorganizes the graph tests into a new tests/graph/ subdirectory, adds a public Graph.handle property, and introduces tests for device-side graph launch.

Closes #1331

Changes

Test Reorganization

• Create new tests/graph/ subdirectory following the pattern of tests/memory_ipc/
• Break apart test_graph.py into logical groupings:
  • test_basic.py: basic graph construction and topology tests
  • test_conditional.py: conditional node tests (if, if-else, switch, while)
  • test_advanced.py: child graphs, update, stream lifetime
  • test_options.py: debug print, complete options, build mode
• Incorporate test_graph_mem.py (renamed to test_capture_alloc.py)
• Add shared kernel compilation helpers in tests/helpers/graph_kernels.py

API Addition

• Add Graph.handle property to expose the underlying CUgraphExec handle
• Follows the existing pattern of Stream.handle and Event.handle
• Forward-compatible with future RAII handle refactoring

Device-Side Graph Launch Tests

• Add test_device_launch.py with tests for device-side graph launch
• Uses cudaGraphLaunch() from device code via cudaStreamGraphTailLaunch
• Requires Hopper (sm_90+) architecture
• Uses cuda.pathfinder.find_nvidia_header_directory() to locate libcudadevrt.a

Test Coverage

• 58 tests total in tests/graph/
• All tests pass on Hopper (H200)
• Device launch tests skip on pre-Hopper architectures