[mono] Preserve signal handlers during crash chaining

[jpnurmi]

This change is part of the effort to fix native crash reporting on Sentry .NET apps on Android (getsentry/sentry-dotnet#3954). We intend to have sentry-native install signal handlers before Mono to capture native crashes. However, mono_handle_native_crash unconditionally resets SIGABRT, SIGILL, SIGCHLD, and SIGQUIT to SIG_DFL even when crash_chaining is enabled, killing the process before the chained handler can run.

Guard the reset with !mono_do_crash_chaining. No effect when crash_chaining is disabled (the default).

Includes an Android functional test (CrashChaining) that verifies signal handlers are preserved during crash chaining.

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[Copilot]

Pull request overview

This PR fixes Mono’s native-crash handling behavior when crash chaining is enabled so that Mono does not reset key signal handlers to SIG_DFL during mono_handle_native_crash, which can otherwise cause secondary signals (notably on Android) to terminate the process before crash chaining completes.

Changes:

• Guard signal-handler resets in mono_handle_native_crash behind !mono_do_crash_chaining.
• Add an Android functional test that installs a pre-Mono SIGSEGV handler, triggers a native crash, and validates SIGABRT was not reset to SIG_DFL.
• Wire the test via the Android app template using an environment-variable gate (TEST_CRASH_CHAINING).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/mono/mono/mini/mini-exceptions.c	Avoids resetting signal handlers to defaults when crash chaining is enabled.
src/tasks/AndroidAppBuilder/Templates/monodroid.c	Adds a crash-chaining functional test path (env-gated) and native test helpers.
src/tests/FunctionalTests/Android/Device_Emulator/CrashChaining/Program.cs	Managed entrypoint invoking the native test and mapping success to exit code 42.
src/tests/FunctionalTests/Android/Device_Emulator/CrashChaining/Android.Device_Emulator.CrashChaining.Test.csproj	New Android functional test project definition + env var injection.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

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