[NativeAOT] Move GC bridge processing from C++ to C#

[Copilot]

Moves the GC bridge processing logic from native C++ to managed C#, reducing native code complexity and eliminating reverse p/invokes.

New C# Implementation

• BridgeProcessing.cs: Port of bridge-processing.cc handling SCC preparation, cross-reference setup, weak/global ref conversion, and cleanup
• ManagedValueManager.cs: Managed background thread replaces native thread; calls BridgeProcessing.Process() directly

Removed C++ Files

• src/native/clr/host/bridge-processing.cc
• src/native/clr/include/host/bridge-processing.hh
• src/native/clr/include/host/bridge-processing-shared.hh
• src/native/nativeaot/host/bridge-processing.cc
• src/native/nativeaot/include/host/bridge-processing.hh

Simplified gc-bridge.hh/cc

• Removed initialize_callback(), bridge_processing(), callback pointers, thread management
• Removed unused includes (<thread>, <shared_mutex>, <unordered_map>)
• Removed clr_initialize_gc_bridge P/Invoke (was never called)

P/Invoke Changes

Direction	Old Mode	New Mode
Native → Managed (reverse)	2 per GC cycle	0
Managed → Native (forward)	0	2 (wait_for_processing, trigger_java_gc)

The native mark_cross_references callback (invoked when managed world is stopped) remains in C++ as required by the runtime.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[NativeAOT] Try to move GC bridge code from C++ to C#</issue_title>
<issue_description>### Android framework version

net11.0-android (Preview)

Affected platform version

.NET 11

Description

The GC bridge for Native AOT and CoreCLR is currently implemented in C++ (#10198). I would like to explore the posibility of moving as much code from native to managed code. All the processing in the background thread could be in managed code (what is now void GCBridge::bridge_processing ()). The only thing that needs to be a native entry point (can be C, doesn't have to be C++) is now void GCBridge::log_mark_cross_references_args_if_enabled (MarkCrossReferencesArgs *args) which runs when the managed world is stopped.

Steps to Reproduce

Did you find any workaround?

No response

Relevant log output

```</issue_description>

<agent_instructions>Most relevant code is in these files:
- src/Mono.Android/Microsoft.Android.Runtime/ManagedValueManager.cs
- src/native/clr/host/gc-bridge.cc
- src/native/clr/host/bridge-processing.cc

The goal is to reduce the amount of C++ code and use C# code which can be better shared between Native AOT and CoreCLR builds of the .NET for Android runtime.</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes [NativeAOT] Try to move GC bridge code from C++ to C# #10747

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