fix: replace malformed XML doc summary for generic ToJsonElement<T>

The auto-generated stub used `<t>` (Roslyn read it as an unclosed XML element)
inside the summary, which surfaces as CS1570 in Release builds and breaks
both the test job and the CodeQL autobuilder. Replace with a hand-written
summary.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nficano

src/Arcp.Core/Envelope/ArcpJson.cs

@@ -44,7 +44,7 @@ public static Envelope Deserialize(ReadOnlySpan<byte> utf8)
     /// <summary>Serialize utf8.</summary>
     public static byte[] SerializeUtf8(Envelope env) => JsonSerializer.SerializeToUtf8Bytes(env, Options);
 
-    /// <summary>To json element<t>.</summary>
+    /// <summary>Serialize a value into a <see cref="JsonElement"/> using ARCP wire options.</summary>
     public static JsonElement ToJsonElement<T>(T value) =>
         JsonSerializer.SerializeToElement(value, Options);
 }