[UR][NativeCPU] Mark IL_VERSION as optional query (#21661)
NativeCPU executes native code directly without intermediate language
(IL) support, so UR_DEVICE_INFO_IL_VERSION query is not applicable.
Changes:
1. Mark IL_VERSION as [optional-query] in device.yml spec
2. Add explanatory comment to ur_api.h describing why this is optional
3. Add IL_VERSION to optional_queries.h list
4. Remove xfails for NativeCPU from SuccessILVersion test
5. Add comment to test explaining why query is optional
The test now correctly handles UNSUPPORTED_ENUMERATION as a valid
response for adapters without IL support.
