fix(ebpf): check bpf_core_read return for task->exit_code

[yairfalse]

Summary

• Check the return value when reading task->exit_code in the container observer.
• Return early when the CO-RE read fails instead of processing the zero initializer.

Why

A failed task->exit_code read left exit_code at zero, making the BPF-side clean-exit filter silently drop the event as if the process exited normally.

Test plan

• cargo check --workspace passes
• eBPF program still compiles with: clang -O2 -g -target bpf -D__TARGET_ARCH_x86 -I ebpf/headers -c ebpf/container_monitor.c -o /tmp/out.o
• failed task->exit_code reads are no longer indistinguishable from clean exits

[yairfalse]

LGTM (review-only — self-authored, can't formally approve).

Minimum-viable correctness fix — exactly the two-line change the bug needed. Failed bpf_core_read is now distinguishable from a clean exit (which the BPF-side filter at line 140 was previously masking).