fix(dispatcher): throw on ambiguous handlers instead of silent .First()

Multiple handlers for the same request were previously resolved via .First(),
with ordering depending on Type.GetInterfaces() order (non-deterministic).
Now ScalarRequestInvoker/StreamRequestInvoker throw InvalidOperationException
listing the candidate handlers when more than one matches.

src/request.dispatcher.tests/_fixtures/DuplicateScalarHandler.cs

@@ -0,0 +1,24 @@
+// Copyright (c) The Geekeey Authors
+// SPDX-License-Identifier: EUPL-1.2
+
+namespace Geekeey.Request.Dispatcher.Tests;
+
+public sealed class DuplicateScalarRequest : IScalarRequest<string>
+{
+}
+
+public sealed class DuplicateScalarHandlerA : IScalarRequestHandler<DuplicateScalarRequest, string>
+{
+	public Task<string> HandleAsync(DuplicateScalarRequest request, CancellationToken cancellationToken)
+	{
+		return Task.FromResult("A");
+	}
+}
+
+public sealed class DuplicateScalarHandlerB : IScalarRequestHandler<DuplicateScalarRequest, string>
+{
+	public Task<string> HandleAsync(DuplicateScalarRequest request, CancellationToken cancellationToken)
+	{
+		return Task.FromResult("B");
+	}
+}

src/request.dispatcher.tests/_fixtures/DuplicateStreamHandler.cs

@@ -0,0 +1,24 @@
+// Copyright (c) The Geekeey Authors
+// SPDX-License-Identifier: EUPL-1.2
+
+namespace Geekeey.Request.Dispatcher.Tests;
+
+public sealed class DuplicateStreamRequest : IStreamRequest<string>
+{
+}
+
+public sealed class DuplicateStreamHandlerA : IStreamRequestHandler<DuplicateStreamRequest, string>
+{
+	public async IAsyncEnumerable<string> HandleAsync(DuplicateStreamRequest request, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken)
+	{
+		yield return "A";
+	}
+}
+
+public sealed class DuplicateStreamHandlerB : IStreamRequestHandler<DuplicateStreamRequest, string>
+{
+	public async IAsyncEnumerable<string> HandleAsync(DuplicateStreamRequest request, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken)
+	{
+		yield return "B";
+	}
+}