feat: Optimize C# binding interop (#488)

- Introduce Utf8Marshaller helpers and SafeHandle wrappers so the managed API centralizes UTF-8 conversions and lifetime management for native pointers.
- Update Engine, Compiler, CompiledPolicy, SchemaRegistry, and TargetRegistry to rely on the new marshaller/safe handles, tightening disposal and reducing transient allocations during interop calls.
- Add allocation guard coverage in Regorus.Tests and report bytes/op in the compiled policy benchmark to surface future regressions.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

bindings/csharp/Regorus/SafeHandles.cs

@@ -0,0 +1,90 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System;
+using System.Runtime.InteropServices;
+using Microsoft.Win32.SafeHandles;
+
+#nullable enable
+namespace Regorus
+{
+    internal sealed class RegorusEngineHandle : SafeHandleZeroOrMinusOneIsInvalid
+    {
+        private RegorusEngineHandle() : base(ownsHandle: true)
+        {
+        }
+
+        internal static RegorusEngineHandle Create()
+        {
+            unsafe
+            {
+                var raw = Internal.API.regorus_engine_new();
+                if (raw is null)
+                {
+                    throw new InvalidOperationException("Failed to create Regorus engine.");
+                }
+
+                var handle = new RegorusEngineHandle();
+                handle.SetHandle((IntPtr)raw);
+                return handle;
+            }
+        }
+
+        internal static RegorusEngineHandle FromPointer(IntPtr pointer)
+        {
+            if (pointer == IntPtr.Zero)
+            {
+                throw new ArgumentException("Pointer cannot be zero.", nameof(pointer));
+            }
+
+            var handle = new RegorusEngineHandle();
+            handle.SetHandle(pointer);
+            return handle;
+        }
+
+        protected override bool ReleaseHandle()
+        {
+            if (!IsInvalid && !IsClosed)
+            {
+                unsafe
+                {
+                    Internal.API.regorus_engine_drop((Internal.RegorusEngine*)handle);
+                }
+                SetHandle(IntPtr.Zero);
+            }
+            return true;
+        }
+    }
+
+    internal sealed class RegorusCompiledPolicyHandle : SafeHandleZeroOrMinusOneIsInvalid
+    {
+        private RegorusCompiledPolicyHandle() : base(ownsHandle: true)
+        {
+        }
+
+        internal static RegorusCompiledPolicyHandle FromPointer(IntPtr pointer)
+        {
+            if (pointer == IntPtr.Zero)
+            {
+                throw new ArgumentException("Pointer cannot be zero.", nameof(pointer));
+            }
+
+            var handle = new RegorusCompiledPolicyHandle();
+            handle.SetHandle(pointer);
+            return handle;
+        }
+
+        protected override bool ReleaseHandle()
+        {
+            if (!IsInvalid && !IsClosed)
+            {
+                unsafe
+                {
+                    Internal.API.regorus_compiled_policy_drop((Internal.RegorusCompiledPolicy*)handle);
+                }
+                SetHandle(IntPtr.Zero);
+            }
+            return true;
+        }
+    }
+}