[FEA]: Support CU_LAUNCH_ATTRIBUTE_PORTABLE_CLUSTER_SIZE_MODE in LaunchConfig

Is this a duplicate?

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Area

cuda.core

Is your feature request related to a problem? Please describe.

LaunchConfig does not expose CU_LAUNCH_ATTRIBUTE_PORTABLE_CLUSTER_SIZE_MODE, which selects portable cluster size mode for a launch.

Parent tracking: #496.

Describe the solution you'd like

• Users can set CU_LAUNCH_ATTRIBUTE_PORTABLE_CLUSTER_SIZE_MODE via LaunchConfig.
• Mapping to the native launch attribute is tested.

Describe alternatives you've considered

Use cuda.bindings.driver (CUlaunchAttribute / CU_LAUNCH_ATTRIBUTE_PORTABLE_CLUSTER_SIZE_MODE) and cuLaunchKernelEx directly.

Downside: leaves cuda.core users without a first-class LaunchConfig API; they must drop to low-level bindings, manage the attribute array themselves, and lose consistency with other LaunchConfig launch attributes (is_cooperative, programmatic_stream_serialization, etc.).

Additional context

Incremental LaunchConfig attribute coverage under #496 (same approach as #1334 for programmatic_stream_serialization).

No example code in the design notes for this attribute.