Python: fix discrimator field for CMC (#8417)

### Motivation and Context

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  1. Why is this change required?
  2. What problem does it solve?
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Turns out we had a mistake in the way CMC discriminates content types,
this PR fixes that

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->
Adds discriminator field to the item in the list rather then the list
itself
Adds additional tests

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

python/.vscode/tasks.json

@@ -122,7 +122,7 @@
                 "pytest",
                 "tests/unit/",
                 "--last-failed",
-                "-v"
+                "-vv"
             ],
             "group": "test",
             "presentation": {

python/semantic_kernel/contents/chat_message_content.py

@@ -3,7 +3,7 @@
 import logging
 from enum import Enum
 from html import unescape
-from typing import Any, ClassVar, Literal, Union, overload
+from typing import Annotated, Any, ClassVar, Literal, overload
 from xml.etree.ElementTree import Element  # nosec
 
 from defusedxml import ElementTree
@@ -26,7 +26,6 @@
 from semantic_kernel.contents.function_result_content import FunctionResultContent
 from semantic_kernel.contents.image_content import ImageContent
 from semantic_kernel.contents.kernel_content import KernelContent
-from semantic_kernel.contents.streaming_text_content import StreamingTextContent
 from semantic_kernel.contents.text_content import TextContent
 from semantic_kernel.contents.utils.author_role import AuthorRole
 from semantic_kernel.contents.utils.finish_reason import FinishReason
@@ -41,15 +40,9 @@
     IMAGE_CONTENT_TAG: ImageContent,
 }
 
-ITEM_TYPES = Union[
-    AnnotationContent,
-    ImageContent,
-    TextContent,
-    StreamingTextContent,
-    FunctionResultContent,
-    FunctionCallContent,
-    FileReferenceContent,
-]
+ITEM_TYPES = (
+    AnnotationContent | ImageContent | TextContent | FunctionResultContent | FunctionCallContent | FileReferenceContent
+)
 
 logger = logging.getLogger(__name__)
 
@@ -78,7 +71,7 @@ class ChatMessageContent(KernelContent):
     tag: ClassVar[str] = CHAT_MESSAGE_CONTENT_TAG
     role: AuthorRole
     name: str | None = None
-    items: list[ITEM_TYPES] = Field(default_factory=list, discriminator=DISCRIMINATOR_FIELD)
+    items: list[Annotated[ITEM_TYPES, Field(..., discriminator=DISCRIMINATOR_FIELD)]] = Field(default_factory=list)
     encoding: str | None = None
     finish_reason: FinishReason | None = None
 

python/tests/unit/contents/test_chat_message_content.py

@@ -284,8 +284,98 @@ def test_cmc_to_dict_keys():
                 "content": [{"type": "text", "text": "Hello, "}, {"type": "text", "text": "world!"}],
             },
         ),
+        (
+            {
+                "role": "user",
+                "items": [
+                    {"content_type": "text", "text": "Hello, "},
+                    {"content_type": "text", "text": "world!"},
+                ],
+            },
+            {
+                "role": "user",
+                "content": [{"type": "text", "text": "Hello, "}, {"type": "text", "text": "world!"}],
+            },
+        ),
+        (
+            {
+                "role": "user",
+                "items": [
+                    {"content_type": "annotation", "file_id": "test"},
+                ],
+            },
+            {
+                "role": "user",
+                "content": [{"type": "text", "text": "test None (Start Index=None->End Index=None)"}],
+            },
+        ),
+        (
+            {
+                "role": "user",
+                "items": [
+                    {"content_type": "file_reference", "file_id": "test"},
+                ],
+            },
+            {
+                "role": "user",
+                "content": [{"file_id": "test"}],
+            },
+        ),
+        (
+            {
+                "role": "user",
+                "items": [
+                    {"content_type": "function_call", "name": "test-test"},
+                ],
+            },
+            {
+                "role": "user",
+                "content": [{"id": None, "type": "function", "function": {"name": "test-test", "arguments": None}}],
+            },
+        ),
+        (
+            {
+                "role": "user",
+                "items": [
+                    {"content_type": "function_call", "name": "test-test"},
+                    {"content_type": "function_result", "name": "test-test", "result": "test", "id": "test"},
+                ],
+            },
+            {
+                "role": "user",
+                "content": [
+                    {"id": None, "type": "function", "function": {"name": "test-test", "arguments": None}},
+                    {"tool_call_id": "test", "content": "test"},
+                ],
+            },
+        ),
+        (
+            {
+                "role": "user",
+                "items": [
+                    {"content_type": "image", "uri": "http://test"},
+                ],
+            },
+            {
+                "role": "user",
+                "content": [{"image_url": {"url": "http://test/"}, "type": "image_url"}],
+            },
+        ),
+    ],
+    ids=[
+        "user_content",
+        "user_with_name",
+        "user_item",
+        "function_call",
+        "function_result",
+        "multiple_items",
+        "multiple_items_serialize",
+        "annotations_serialize",
+        "file_reference_serialize",
+        "function_call_serialize",
+        "function_result_serialize",
+        "image_serialize",
     ],
-    ids=["user_content", "user_with_name", "user_item", "function_call", "function_result", "multiple_items"],
 )
 def test_cmc_to_dict_items(input_args, expected_dict):
     message = ChatMessageContent(**input_args)