test(generate): lock in getgid null enter and UNCLASSIFIED ret

Audit of getgid(2) found its classification already correct: family
Process, enter KindNull (gid_t getgid(void) takes no arguments), exit
KindRet with UNCLASSIFIED ret_type (returns a gid, never a byte count,
and always succeeds). Family, kind, generated C handler, and docs all
matched its no-arg id-returning siblings getuid/geteuid/getegid/getpid/
gettid/getppid, so no implementation or doc changes were needed.

Add two dedicated lock-in tests using the real tracepoint fields,
mirroring the setuid/setpgid audit pattern, so a stray reclassification
of getgid trips a test:
- TestClassifyGetgidNullEnter: enter is KindNull, no path/fd capture.
- TestClassifyExitGetgidUnclassifiedRet: exit is KindRet, UNCLASSIFIED.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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