Fix DuckTypeFieldIsReadonlyException in RabbitMQ v7+ auto-instrumentation (#8006)

## Summary of changes

Fixes a `DuckTypeFieldIsReadonlyException` happening in the RabbitMQ v7+
instrumentation when `BasicPublish` is called with `CachedString`s.

## Reason for change


https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/94147996821f6a77bed0e77fd1886e75b08a31f9/projects/RabbitMQ.Client/CachedString.cs#L16

is defined as `readonly`

Our DuckType had was a `get/set` property.

When calling `BasicPublish` with `CachedString` we'd get the following
error:

```
2025-12-23 09:52:24.105 -05:00 [ERR] Exception occurred when calling the CallTarget integration continuation. System.TypeInitializationException: The type initializer for 'Datadog.Trace.ClrProfiler.CallTarget.Handlers.BeginMethodHandler`8' threw an exception.
 ---> Datadog.Trace.ClrProfiler.CallTarget.CallTargetInvokerException: The field 'Value' is marked as readonly, you should remove the setter from the base type class or interface.
 ---> Datadog.Trace.DuckTyping.DuckTypeFieldIsReadonlyException: The field 'Value' is marked as readonly, you should remove the setter from the base type class or interface.
   at Datadog.Trace.DuckTyping.DuckTypeFieldIsReadonlyException.Throw(FieldInfo field) in C:\Users\steven.bouwkamp\source\repos\dd-trace-dotnet2\tracer\src\Datadog.Trace\DuckTyping\DuckTypeExceptions.cs:line 171
   at Datadog.Trace.DuckTyping.DuckType.CreateProperties(TypeBuilder proxyTypeBuilder, Type proxyDefinitionType, Type targetType, FieldInfo instanceField) in C:\Users\steven.bouwkamp\source\repos\dd-trace-dotnet2\tracer\src\Datadog.Trace\DuckTyping\DuckType.cs:line 774
   at Datadog.Trace.DuckTyping.DuckType.CreateProxyType(Type proxyDefinitionType, Type targetType, Boolean dryRun) in C:\Users\steven.bouwkamp\source\repos\dd-trace-dotnet2\tracer\src\Datadog.Trace\DuckTyping\DuckType.cs:line 215
--- End of stack trace from previous location ---
   at Datadog.Trace.ClrProfiler.CallTarget.Handlers.IntegrationMapper.CreateBeginMethodDelegate(Type integrationType, Type targetType, Type[] argumentsTypes) in C:\Users\steven.bouwkamp\source\repos\dd-trace-dotnet2\tracer\src\Datadog.Trace\ClrProfiler\CallTarget\Handlers\IntegrationMapper.cs:line 142
   at Datadog.Trace.ClrProfiler.CallTarget.Handlers.BeginMethodHandler`8..cctor() in C:\Users\steven.bouwkamp\source\repos\dd-trace-dotnet2\tracer\src\Datadog.Trace\ClrProfiler\CallTarget\Handlers\BeginMethodHandler`6.cs:line 28
   --- End of inner exception stack trace ---
   at Datadog.Trace.ClrProfiler.CallTarget.Handlers.BeginMethodHandler`8..cctor() in C:\Users\steven.bouwkamp\source\repos\dd-trace-dotnet2\tracer\src\Datadog.Trace\ClrProfiler\CallTarget\Handlers\BeginMethodHandler`6.cs:line 35
   --- End of inner exception stack trace ---
   at RabbitMQ.Client.Impl.Channel.BasicPublishAsync[TProperties](CachedString exchange, CachedString routingKey, Boolean mandatory, TProperties basicProperties, ReadOnlyMemory`1 body, CancellationToken cancellationToken)
 { MachineName: ".", Process: "[68600 Samples.RabbitMQ]", AppDomain: "[1 Samples.RabbitMQ]", AssemblyLoadContext: "\"\" Datadog.Trace.ClrProfiler.Managed.Loader.ManagedProfilerAssemblyLoadContext #2", TracerVersion: "3.35.0.0" }
```

## Implementation details

Removed the `set`.

## Test coverage

So, I updated the sample application to make a call with `CachedString`s
on V7+ _instead_ of the normal `string` calls. So no new spans, we would
expect the spans to be the same and they are.

Without this fix (removing the `set`) the tests fail as the snapshots
are different for V7. I didn't think it was super valuable to update the
sample application to call both versions of `BasicPublish` for V7 (one
with `CachedString` and one without) as
`BasicPublishAsyncCachedStringsIntegration` just calls `return
BasicPublishAsyncIntegration.OnMethodBegin` so 🤷

## Other details
<!-- Fixes #{issue} -->

Fixes [this error identified by Error
Tracking](https://app.datadoghq.com/error-tracking/issue/b5bb096c-6bd4-11f0-adde-da7ad0900002)

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Author: bouwkast

tracer/src/Datadog.Trace/ClrProfiler/AutoInstrumentation/RabbitMQ/BasicPublishAsyncCachedStringsIntegration.cs

@@ -55,8 +55,8 @@ internal static CallTargetState OnMethodBegin<TTarget, TExchange, TRoutingKey, T
 internal interface ICachedStringProxy : IDuckType
 {
     /// <summary>
-    /// Gets or sets a value of System.String
+    /// Gets the value of System.String
     /// </summary>
     [DuckField(Name = "Value")]
-    string Value { get; set; }
+    string Value { get; }
 }

tracer/test/test-applications/integrations/Samples.RabbitMQ/Program.cs

@@ -135,7 +135,19 @@ private static async Task PublishAndGet(IRabbitChannel channel, string consumerT
                 string message = $"{messagePrefix} - Message";
                 var body = Encoding.UTF8.GetBytes(message);
 
+#if RABBITMQ_7_0
+                // Use CachedString parameters to trigger BasicPublishAsyncCachedStringsIntegration
+                var cachedExchange = new CachedString(publishExchangeName);
+                var cachedRoutingKey = new CachedString(publishRoutingKey);
+                await channel.BasicPublishAsync(
+                    exchange: cachedExchange,
+                    routingKey: cachedRoutingKey,
+                    mandatory: false,
+                    basicProperties: properties,
+                    body: body);
+#else
                 await Helper.BasicPublishAsync(channel, publishExchangeName, publishRoutingKey, body, properties);
+#endif
 
                 Console.WriteLine($"BasicPublish - Sent message: {message}");
             }
@@ -180,9 +192,19 @@ private static async Task Send(IConnection connection, string consumerType)
                         string message = $"Send - Message #{i}";
                         var body = Encoding.UTF8.GetBytes(message);
 
+#if RABBITMQ_7_0
+                        // Use CachedString parameters to trigger BasicPublishAsyncCachedStringsIntegration
+                        var cachedExchange = new CachedString("");
+                        var cachedRoutingKey = new CachedString("hello");
+                        await channel.BasicPublishAsync(
+                            exchange: cachedExchange,
+                            routingKey: cachedRoutingKey,
+                            body: body);
+#else
                         await Helper.BasicPublishAsync (channel, exchange: "",
                                              routingKey: "hello",
                                              body: body);
+#endif
                         Console.WriteLine("[Send] - [x] Sent \"{0}\"", message);