fix(android): Preserve NDK stack on tombstone merge (JAVA-645) [on hold]

[runningcode]

Warning

On hold — premise corrected by a control experiment. A genuine native fault (null-deref inside libnative-sample.so) symbolicates to native-sample.cpp:13 on main without this change, and also with it. Native symbolication / uploaded symbols were never broken — the original "unreadable native crash" was an artifact of the sample's raise(SIGSEGV)-from-JNI crash (a managed-origin crash whose tombstone stack is the ART/OAT managed chain).

What this change actually does for a real native crash: main keeps the tombstone's full mixed stack (symbolicated native crash frame + the obfuscated managed call chain); this PR uses sentry-native's native-only crashing thread, which drops that call-chain context. So it is neutral-to-worse for genuine native crashes, not the win described below.

The higher-value fix is to deobfuscate the managed frames in the merged native event (they stay obfuscated only because the event platform == "native"). See JAVA-645 for the reframed analysis. Holding this PR pending that decision.

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📜 Description

When io.sentry.tombstone.enable=true, a native crash that is also captured by sentry-native is reported through the ApplicationExitInfo tombstone path (mechanism: TombstoneMerged). TombstoneIntegration.mergeNativeCrashes kept the Native SDK event's contexts but overwrote its exceptions, threads, and debugMeta with the tombstone's. For JNI-raised crashes the tombstone's crashing-thread stack is the ART-unwound managed stack (OAT/VDEX frames), which cannot be symbolicated (native) or ProGuard-deobfuscated, so the merged event became unreadable.

This change keeps the Native SDK's crashing-thread stack-trace, threads and debug-meta — which are symbolicated against the uploaded debug files — and only enriches the Native SDK exception with the tombstone's signal metadata (and flags the merge via the mechanism type). It falls back to the tombstone's data when the Native SDK event carries no exception.

💡 Motivation and Context

Native crashes on affected builds were effectively unreadable: obfuscated managed frames plus a couple of system frames, with no app source lines. Uploading native symbols and the ProGuard mapping did not help, because the frames that would consume them were discarded/replaced.

Evidence from a raw TombstoneMerged event JSON:

• Server symbolication works — libsentry.so shows debug_status: "found", and sentry-native's own worker threads symbolicate to source (sentry_backend_inproc.c:1238, sentry_batcher.c:339).
• The crashing thread's frames were all base.vdex/base.odex/boot.oat with symbolicator_status: "unknown_image", retaining obfuscated names; libnative-sample.so was debug_status: "unused".
• The same event still contained fully-symbolicated sentry-native worker threads while the crashing thread carried the tombstone's managed stack — direct evidence the merge overwrote the good crashing-thread data.

Reproduced on Android 12, 16 and 17 (Pixel 3, Galaxy A55, Pixel 10) and SDK 8.48.0 / 8.49.0. The sentry monolith and symbolicator behave correctly — the fix is entirely client-side.

Fixes JAVA-645

💚 How did you test it?

Manually, with the sentry-samples-android release build (SAGP enabled, native symbols + ProGuard mapping uploaded):

• Before: io.sentry.tombstone.enable=true → TombstoneMerged, unsymbolicated / obfuscated crashing stack.
• With io.sentry.tombstone.enable=false → mechanism: signalhandler, and the pipeline symbolicates — confirming the merge was the cause.

Automated tests for mergeNativeCrashes still need to be added/updated (see Next steps) — this is a draft.

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

• Add/adjust TombstoneIntegrationTest coverage asserting the Native SDK crashing-thread stack/threads/debug-meta are preserved and only the mechanism/signal metadata is merged.
• Native-team review of the merge policy — in particular whether tombstone-only (no matching NDK event) crashes should retain managed frames tagged platform: java so ProGuard deobfuscation applies.

[linear-code]

JAVA-645

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.49.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	327.96 ms	372.89 ms	44.93 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
b193867	319.59 ms	403.09 ms	83.50 ms
2124a46	319.19 ms	415.04 ms	95.85 ms
a5ab36f	316.83 ms	394.54 ms	77.71 ms
dba088c	320.59 ms	361.29 ms	40.70 ms
4e3e79d	328.10 ms	395.64 ms	67.54 ms
abfcc92	309.54 ms	380.32 ms	70.78 ms
539ca63	313.51 ms	355.43 ms	41.92 ms
3d205d0	352.15 ms	432.53 ms	80.38 ms
bb0ff41	312.86 ms	363.78 ms	50.92 ms
2195398	319.02 ms	342.38 ms	23.36 ms

App size

Revision	Plain	With Sentry	Diff
b193867	1.58 MiB	2.19 MiB	620.00 KiB
2124a46	1.58 MiB	2.12 MiB	551.51 KiB
a5ab36f	1.58 MiB	2.12 MiB	555.26 KiB
dba088c	1.58 MiB	2.13 MiB	558.99 KiB
4e3e79d	0 B	0 B	0 B
abfcc92	1.58 MiB	2.13 MiB	557.31 KiB
539ca63	1.58 MiB	2.12 MiB	551.41 KiB
3d205d0	1.58 MiB	2.10 MiB	532.97 KiB
bb0ff41	0 B	0 B	0 B
2195398	0 B	0 B	0 B