feat(android): allow NDK preload for .NET/CoreCLR

[jpnurmi]

.NET 10.0 added support for CoreCLR on Android (experimental). Unlike with the Mono runtime, where we're stuck with CHAIN_AT_START (dotnet/runtime#125835), with CoreCLR we can already reorder the signal handlers to the preferred order:

┌───────────┐     ┌───────────────┐     ┌────────────────────┐
│ .NET/Mono │────>│ Sentry Native │────>│ System (debuggerd) │
└───────────┘     └───────────────┘     └────────────────────┘

This gives the .NET runtime a chance to process managed .NET exceptions without Sentry Native stepping on its toes. This is much safer than CHAIN_AT_START, and will be automatically enabled (io.sentry.ndk.preload=true) by Sentry.NET when targeting Android .NET 10.0 or later with CoreCLR.

Changes

Add sentry__backend_preload() for installing signal handlers before sentry_init(). For the inproc backend on Unix/Android, this allows establishing the correct signal handler chain order when another runtime (e.g. Mono/.NET) installs its own handlers later.

• Backend dispatch: sentry__backend_preload() (no-op for non-inproc)
• Extracted install_signal_handlers() with idempotency flag
• Preload fallthrough: if handler thread not ready, invoke previous handler
• NDK: SentryNdkPreloadProvider content provider with high init order for early signal handler installation before the CoreCLR runtime
• NDK: Java/JNI preload() method on SentryNdk

See also

• Original issue: Prevent Native/Cocoa SDKs from capturing errors that also get captured by .NET sentry-dotnet#3954
• Signal handler re-order for iOS: fix: AOT interop with managed .NET runtimes sentry-cocoa#6193
• Chain-at-start alternative for Android on Mono: fix(android): mask signal during CHAIN_AT_START to survive chained Mono handler re-raises #1572
• Mono fix for future reorder: [mono] Restore signal handlers during crash chaining dotnet/runtime#125835

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