feat(cuda.core): support CU_LAUNCH_ATTRIBUTE_PRIORITY in LaunchConfig

[ichensun]

Description

issues #2631

Adds a priority attribute to LaunchConfig that maps to
CU_LAUNCH_ATTRIBUTE_PRIORITY,
following the same pattern used for programmatic_stream_serialization (#1334).

• priority: int | None = None (default None) — when omitted, the launch
  uses the stream's priority, matching existing LaunchConfig attribute
  conventions.
• Lower numbers represent higher priority; 0 and negative values are valid,
  meaningful priorities, not sentinels for "unset". Because of this, the
  attribute is stored as a nullable object (not a C int) internally, and
  both native-config conversion paths (LaunchConfig._to_native_launch_config
  and the module-level _to_native_launch_config) check is not None rather
  than truthiness, so priority=0 is set correctly instead of being dropped.
• No additional device-capability validation is added (unlike
  is_cooperative), since the driver accepts any priority within the
  device's stream-priority range and clamps/errors on its own.

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