fix(a2a-client): read 0.3-shaped file parts and report parts the SDK can't read

The 1.0 SDK ignores unknown part fields, so a file part shaped {kind:file, file:{bytes|uri,
name, mimeType}} arrived as an empty part and vanished. A2AWireCompat rewrites such parts
to {raw|url, filename, mediaType} in JSON and SSE responses before the SDK reads them, and
marks any part it still can't read so the result lists it with its field names.

tests/Volcanic.AgenticCore.Extensions.Tests/A2A/A2AWireCompatTests.cs

@@ -0,0 +1,93 @@
+using System.Net;
+using System.Text;
+using A2A;
+using Microsoft.Extensions.Logging.Abstractions;
+using Volcanic.AgenticCore.Extensions.A2A;
+
+namespace Volcanic.AgenticCore.Extensions.Tests.A2A;
+
+public sealed class A2AWireCompatTests : IDisposable
+{
+    private readonly string _ws = Path.Combine(Path.GetTempPath(), "a2a-wire", Guid.NewGuid().ToString("N"));
+
+    private static readonly string Skill = Convert.ToBase64String(Encoding.UTF8.GetBytes("# skill"));
+
+    private static string TaskJson(string parts) => $$"""
+        {"id":"t-1","contextId":"c-1","status":{"state":"TASK_STATE_COMPLETED"},
+         "artifacts":[{"artifactId":"response","parts":[{"kind":"text","text":"Here you go"}]},
+                      {"artifactId":"files","parts":[{{parts}}]}]}
+        """;
+
+    private static IA2AClient Client(Func<HttpRequestMessage, HttpResponseMessage> respond) =>
+        new A2AClient(new Uri("https://agent.example/a2a"),
+            new HttpClient(new A2AWireCompat(new Fake(respond), () => NullLogger.Instance)));
+
+    private static HttpResponseMessage Json(string result) => new(HttpStatusCode.OK)
+    {
+        Content = new StringContent($$"""{"jsonrpc":"2.0","id":1,"result":{{result}}}""", Encoding.UTF8, "application/json"),
+    };
+
+    [Fact]
+    public async Task GetTask_V03FileParts_BecomeRawAndUrl()
+    {
+        var client = Client(_ => Json(TaskJson(
+            """{"kind":"file","file":{"bytes":"SKILL","name":"SKILL.md","mimeType":"text/markdown"}},""".Replace("SKILL\"", Skill + "\"") +
+            """{"kind":"file","file":{"uri":"https://cdn.example/crm.tar.gz","name":"crm.tar.gz","mimeType":"application/gzip"}}""")));
+
+        var task = await client.GetTaskAsync(new GetTaskRequest { Id = "t-1" });
+
+        var parts = task.Artifacts![1].Parts;
+        Assert.Equal("# skill", Encoding.UTF8.GetString(parts[0].Raw!));
+        Assert.Equal(("SKILL.md", "text/markdown"), (parts[0].Filename!, parts[0].MediaType!));
+        Assert.Equal("https://cdn.example/crm.tar.gz", parts[1].Url);
+        Assert.Equal("crm.tar.gz", parts[1].Filename);
+        Assert.Equal("Here you go", A2AResults.FinalText(task));
+    }
+
+    [Fact]
+    public async Task UnknownPart_ReportedInFiles_V1PartsUntouched()
+    {
+        var client = Client(_ => Json(TaskJson(
+            "{\"raw\":\"" + Skill + "\",\"filename\":\"SKILL.md\"}," + """{"kind":"blob","blob":{"content":"x","title":"y"}}""")));
+        var task = await client.GetTaskAsync(new GetTaskRequest { Id = "t-1" });
+
+        var files = await A2AFiles.SaveAsync(_ws, task, null, new Fake(_ => new HttpResponseMessage(HttpStatusCode.NotFound)), new A2AClientSettings(), CancellationToken.None);
+
+        Assert.Equal("inbox/a2a/t-1/SKILL.md", files[0].Path);
+        Assert.Equal("not understood (fields: kind, blob{content, title})", files[1].Problem);
+    }
+
+    [Fact]
+    public async Task Subscribe_SseEvents_Rewritten()
+    {
+        var client = Client(_ => new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = new StringContent(
+                "event: message\n" +
+                "data: " + ("""{"jsonrpc":"2.0","id":1,"result":{"task":""" +
+                            TaskJson("{\"kind\":\"file\",\"file\":{\"bytes\":\"" + Skill + "\",\"name\":\"SKILL.md\"}}") + "}}").Replace("\n", "") + "\n\n",
+                Encoding.UTF8, "text/event-stream"),
+        });
+        var events = new List<StreamResponse>();
+        await foreach (var ev in client.SubscribeToTaskAsync(new SubscribeToTaskRequest { Id = "t-1" })) events.Add(ev);
+
+        var part = Assert.Single(events).Task!.Artifacts![1].Parts.Single();
+        Assert.Equal("# skill", Encoding.UTF8.GetString(part.Raw!));
+        Assert.Equal("SKILL.md", part.Filename);
+    }
+
+    [Fact]
+    public void Rewrite_NoParts_LeavesBodyAlone()
+    {
+        Assert.Null(A2AWireCompat.Rewrite("""{"name":"card"}""", NullLogger.Instance));
+        Assert.Null(A2AWireCompat.Rewrite("""{"parts":[{"text":"hi"}]}""", NullLogger.Instance));
+    }
+
+    public void Dispose() { try { Directory.Delete(_ws, true); } catch (IOException) { } }
+
+    private sealed class Fake(Func<HttpRequestMessage, HttpResponseMessage> respond) : HttpMessageHandler
+    {
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) =>
+            Task.FromResult(respond(request));
+    }
+}