When ebpfcore is not running, ebpf_program_load() returns EBPF_INVALID_FD

When ebpfcore is not running, `ebpf_program_load()` API returns `EBPF_INVALID_FD`, which is confusing to debug.

In the case when ebpfcore is not running, the API should return a clearer error code.

Start by tracing the ebpf_program_load() API path for the case where ebpfcore is unavailable, and find where that condition becomes EBPF_INVALID_FD. Identify the existing error-code tests or test entry points for this API. Done means the unavailable-service case returns a clearer, distinct error code and the behavior is covered by a test.

Written by the indexing model from the issue text.