fix(native): use breakpad's 0x40000000 MD_CONTEXT_ARM flag

The ARM32 write_thread_context branch was emitting
context_flags = 0x00200003 (Microsoft Windows CONTEXT_ARM | CONTROL |
INTEGER). That matched the pattern the other arches use in this file,
but ARM32 is the one arch where breakpad explicitly diverges from
Microsoft's numbering, with a comment in minidump_cpu_arm.h:

  // This value was chosen to avoid likely conflicts with MD_CONTEXT_*
  // for other CPUs.
  #define MD_CONTEXT_ARM 0x40000000

rust-minidump (Sentry's ingest reader) keys off the breakpad value:

  https://github.com/rust-minidump/rust-minidump/blob/d4fefc765aad35b3bef569d53c1680eadab5a268/minidump-common/src/format.rs#L801-L804

With 0x00200000, the parser wouldn't identify the stream as an ARM
context and registers would be dropped from the crash report. Switch
to 0x40000003 to match breakpad / rust-minidump / crashpad (all of
which use 0x40000000 for ARM specifically).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jpnurmi

src/backends/native/minidump/sentry_minidump_linux.c

@@ -781,8 +781,9 @@ write_thread_context(
     (void)tid; // Unused on ARM32 - no VFP capture implemented yet
 
     minidump_context_arm_t context = { 0 };
-    // CONTEXT_ARM | CONTEXT_CONTROL | CONTEXT_INTEGER
-    context.context_flags = 0x00200003;
+    // MD_CONTEXT_ARM | CONTROL | INTEGER — breakpad-style 0x40000000 base
+    // (not Microsoft's 0x00200000; rust-minidump keys off the breakpad value)
+    context.context_flags = 0x40000003;
 
     // Copy general purpose registers R0-R10 from Linux ucontext
     context.r[0] = uctx->uc_mcontext.arm_r0;