feat(samples): Make Native Crash button fault inside app native code (#5773)

The Native Crash button called raise(SIGSEGV), which faults inside libc and, for a JNI-originated crash, does not exercise symbolication of the app's own native code. Point it at a null-deref inside a named function (trigger_null_deref) instead, so the crashing frame resolves to a real symbol + source line and native symbolication can be verified from the sample.

Ref JAVA-645


Claude-Session: https://claude.ai/code/session_01EmE8hdaj9H9K61opK2PZ6U

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>