name the field when sampling content is missing (#689)

Follow-up to [this thread](https://github.com/dart-lang/ai/pull/685#discussion_r4063946725) on #685.

A `SamplingMessage` without `content` now throws an `ArgumentError` naming the field, instead of a `TypeError` from the cast. Its wording follows the other missing-field checks under `lib/src/api/`.

pkgs/dart_mcp/CHANGELOG.md

diff --git a/pkgs/dart_mcp/CHANGELOG.md b/pkgs/dart_mcp/CHANGELOG.md
index 8db24e2..bd3b70d 100644
--- a/pkgs/dart_mcp/CHANGELOG.md
+++ b/pkgs/dart_mcp/CHANGELOG.md
@@ -151,8 +151,9 @@
   - `SamplingMessage.content` and `CreateMessageResult.content` now read
     `List<SamplingMessageContentBlock>` instead of `Content`. The schema
     allows one block or a list of them under `content`; both shapes read as a
-    list, and one block still goes on the wire as that block. `TextContent`,
-    `ImageContent` and `AudioContent` implement both types.
+    list, and one block still goes on the wire as that block. A missing
+    `content` throws an `ArgumentError`. `TextContent`, `ImageContent` and
+    `AudioContent` implement both types.
     `ToolUseContent` and `ToolResultContent` implement only the new one,
     keeping a plain `tools/call` result from carrying tool content by
     accident. `ToolResultContent.content` still reads `List<Content>`,

pkgs/dart_mcp/lib/src/api/sampling.dart

diff --git a/pkgs/dart_mcp/lib/src/api/sampling.dart b/pkgs/dart_mcp/lib/src/api/sampling.dart
index f381b7a..ec5f2be 100644
--- a/pkgs/dart_mcp/lib/src/api/sampling.dart
+++ b/pkgs/dart_mcp/lib/src/api/sampling.dart
@@ -178,6 +178,9 @@
   /// read as a list here. One block comes back as a single-element list.
   List<SamplingMessageContentBlock> get content {
     final content = _value[Keys.content];
+    if (content == null) {
+      throw ArgumentError('Missing ${Keys.content} field in $SamplingMessage.');
+    }
     if (content is List) {
       return content.cast<SamplingMessageContentBlock>();
     }

pkgs/dart_mcp/test/api/sampling_test.dart

diff --git a/pkgs/dart_mcp/test/api/sampling_test.dart b/pkgs/dart_mcp/test/api/sampling_test.dart
index e3dfde6..e2d9d18 100644
--- a/pkgs/dart_mcp/test/api/sampling_test.dart
+++ b/pkgs/dart_mcp/test/api/sampling_test.dart
@@ -81,6 +81,13 @@
       expect((parsed.content.single as TextContent).text, 'merhaba');
     });
 
+    test('SamplingMessage throws when content is missing', () {
+      expect(
+        () => SamplingMessage.fromMap({'role': 'user'}).content,
+        throwsArgumentError,
+      );
+    });
+
     test('CreateMessageResult accepts a SamplingMessageContentBlock', () {
       final block = SamplingMessageContentBlock.toolResult(
         toolUseId: 'call-2',