Reduce generated JNI callback IL size (#1452)

Fixes #1359

## Summary

This reduces generated JNI callback IL size by moving the repeated marshal-boundary wrapper code out of every generated callback and into generated helper overloads.

Generated callbacks now have two pieces:

- `n_*`: a small native-entry thunk that forwards JNI arguments to a generated safe invoker.
- `__n_*`: the actual callback body that performs `GetObject`, argument conversion, user call/property access, cleanup, and return marshaling.

`Java.Interop.JniMarshal` now provides shared `SafeInvokeAction` and `SafeInvokeFunc` overloads. These helpers centralize:

- `JniEnvironment.BeginMarshalMethod(...)`
- `try` / `catch (Exception)` / `finally`
- `JniRuntime.OnUserUnhandledException(...)`
- `JniEnvironment.EndMarshalMethod(...)`

The helper shape is intentionally C#-only and uses managed function pointers with the function pointer as the final argument:

```csharp
Java.Interop.JniMarshal.SafeInvokeAction (jnienv, native__this, arg0, arg1, &__n_Foo);
Java.Interop.JniMarshal.SafeInvokeFunc (jnienv, native__this, arg0, &__n_Bar);
```

## Design notes

During prototyping I tested several alternatives:

- explicit IL `tail.` thunks
- C# forwarding patterns that might encourage tailcalls
- NativeAOT output for C# forwarding patterns
- function pointer first vs. function pointer last

The useful findings were:

- C# and NativeAOT do not reliably emit tailcalls for these forwarding patterns.
- Explicit `tail.` can help low-arity compiled IL thunks, but has severe arity/ABI cliffs. On arm64, mixed signatures around 8+ args fell back to helper-based tailcalls and became ~4x slower than normal calls.
- Function-pointer-last preserves JNI argument register/stack placement better than function-pointer-first.

So this PR deliberately avoids explicit `tail.` and uses the safer C# helper shape with JNI args first and fnptr last. The safe invokers live in `Java.Interop.JniMarshal` rather than generated binding output, so each binding assembly only emits the small `n_*`/`__n_*` split and does not get its own copy of the helper methods.


## DebuggerDisableUserUnhandledExceptions placement

This PR does not remove `[global::System.Diagnostics.DebuggerDisableUserUnhandledExceptions]` from the marshal exception boundary. It moves the attribute to the shared `Java.Interop.JniMarshal.SafeInvokeAction` / `SafeInvokeFunc` helpers because those helpers now own the `try` / `catch (Exception)` block and call `OnUserUnhandledException(...)`.

The generated `n_*` methods are now only forwarding thunks and no longer catch user exceptions. Keeping the attribute on every `n_*` thunk would be redundant for debugger behavior and would add back per-callback metadata/IL that this change is trying to remove. The attribute follows the catch block.

## Mono.Android size measurements

Measured by building Mono.Android from a dotnet/android worktree with this Java.Interop generator patch applied.

The shared-helper version has no generated `__JniMarshalMethodHelper` copies (`0` matches in generated mcw output). Generated callbacks call `Java.Interop.JniMarshal.SafeInvoke*` instead (`29,967` call sites in Mono.Android generated output).

### Release

| Artifact | Baseline | Patched | Delta |
|---|---:|---:|---:|
| Runtime `Mono.Android.dll` | 42,798,592 | 42,121,728 | **-676,864 bytes** |
| Ref `Mono.Android.dll` | 19,295,744 | 19,339,776 | +44,032 bytes |
| `Mono.Android.pdb` | 32,553,308 | 30,690,252 | **-1,863,056 bytes** |
| Generated mcw `.cs` total | 29,234,484 | 28,508,893 | **-725,591 bytes** |

### Debug

| Artifact | Baseline | Patched | Delta |
|---|---:|---:|---:|
| Runtime `Mono.Android.dll` | 46,168,064 | 45,507,584 | **-660,480 bytes** |
| Ref `Mono.Android.dll` | 19,294,720 | 19,339,264 | +44,544 bytes |
| `Mono.Android.pdb` | 38,057,736 | 35,815,552 | **-2,242,184 bytes** |
| Generated mcw `.cs` total | 29,234,484 | 28,508,893 | **-725,591 bytes** |

Compared with the earlier generated-helper prototype, moving the helper into `Java.Interop.JniMarshal` saves an additional ~11.5 KiB in runtime `Mono.Android.dll`, ~8.5 KiB in ref `Mono.Android.dll`, and ~6.8 KiB of generated mcw source.

Measurement notes are saved locally at:

`~/.copilot/session-state/fd1f37cb-e759-40da-8b40-c668a63336f6/files/android-issue-1359-shared-helper-measure-summary.txt`

## Validation

- `dotnet build tools/generator/generator.csproj -v:minimal`
- `dotnet test tests/generator-Tests/generator-Tests.csproj -v:minimal`
  - 455 passed, 0 failed

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: simonrozsival

external/Java.Interop/src/Java.Interop/Java.Interop.csproj

@@ -60,6 +60,10 @@
       <Generator>TextTemplatingFileGenerator</Generator>
       <LastGenOutput>JniPeerMembers.JniInstanceMethods_Invoke.cs</LastGenOutput>
     </None>
+    <None Include="Java.Interop\JniMarshal.SafeInvoke.tt">
+      <Generator>TextTemplatingFileGenerator</Generator>
+      <LastGenOutput>JniMarshal.SafeInvoke.cs</LastGenOutput>
+    </None>
     <None Include="Java.Interop\JniPeerMembers.JniFields.tt">
       <Generator>TextTemplatingFileGenerator</Generator>
       <LastGenOutput>JniPeerMembers.JniFields.cs</LastGenOutput>
@@ -86,6 +90,9 @@
     <Compile Condition=" '$(EnableDefaultCompileItems)' == 'true' " Update="Java.Interop\JniPeerMembers.JniInstanceMethods_Invoke.cs">
       <DependentUpon>JniPeerMembers.JniInstanceMethods_Invoke.tt</DependentUpon>
     </Compile>
+    <Compile Condition=" '$(EnableDefaultCompileItems)' == 'true' " Update="Java.Interop\JniMarshal.SafeInvoke.cs">
+      <DependentUpon>JniMarshal.SafeInvoke.tt</DependentUpon>
+    </Compile>
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.CodeAnalysis.PublicApiAnalyzers" Version="4.14.0">