Add missing exception documentation to public API surface (#1103)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol.Core/Protocol/ElicitRequestParams.cs

@@ -20,6 +20,7 @@ public sealed class ElicitRequestParams : RequestParams
     ///   <item><description><b>url</b>: Client navigates user to a URL for out-of-band interaction. Sensitive data is not exposed to the client.</description></item>
     /// </list>
     /// </remarks>
+    /// <exception cref="ArgumentException">The value is not "form" or "url".</exception>
     [JsonPropertyName("mode")]
     [field: MaybeNull]
     public string Mode

src/ModelContextProtocol.Core/Protocol/Tool.cs

@@ -39,6 +39,7 @@ public sealed class Tool : IBaseMetadata
     /// <summary>
     /// Gets or sets a JSON Schema object defining the expected parameters for the tool.
     /// </summary>
+    /// <exception cref="ArgumentException">The value is not a valid MCP tool JSON schema.</exception>
     /// <remarks>
     /// <para>
     /// The schema must be a valid JSON Schema object with the "type" property set to "object".
@@ -73,6 +74,7 @@ public sealed class Tool : IBaseMetadata
     /// <summary>
     /// Gets or sets a JSON Schema object defining the expected structured outputs for the tool.
     /// </summary>
+    /// <exception cref="ArgumentException">The value is not a valid MCP tool JSON schema.</exception>
     /// <remarks>
     /// <para>
     /// The schema must be a valid JSON Schema object with the "type" property set to "object".