multiple: permit optional fields on multimodal content blocks (#30887)

Instead of stuffing provider-specific fields in `metadata`, they can go
directly on the content block.

libs/core/langchain_core/messages/content_blocks.py

@@ -6,13 +6,11 @@ from pydantic import TypeAdapter, ValidationError
 from typing_extensions import NotRequired, TypedDict
 
 
-class BaseDataContentBlock(TypedDict):
+class BaseDataContentBlock(TypedDict, total=False):
     """Base class for data content blocks."""
 
     mime_type: NotRequired[str]
     """MIME type of the content block (if needed)."""
-    metadata: NotRequired[dict]
-    """Provider-specific metadata such as citations or filenames."""
 
 
 class URLContentBlock(BaseDataContentBlock):

libs/core/tests/unit_tests/test_messages.py

@@ -1107,6 +1107,15 @@ def test_is_data_content_block() -> None:
             "mime_type": "image/jpeg",
         }
     )
+    assert is_data_content_block(
+        {
+            "type": "image",
+            "source_type": "base64",
+            "data": "<base64 data>",
+            "mime_type": "image/jpeg",
+            "cache_control": {"type": "ephemeral"},
+        }
+    )
     assert is_data_content_block(
         {
             "type": "image",
@@ -1148,7 +1157,7 @@ def test_convert_to_openai_image_block() -> None:
         "type": "image",
         "source_type": "url",
         "url": "https://...",
-        "metadata": {"cache_control": {"type": "ephemeral"}},
+        "cache_control": {"type": "ephemeral"},
     }
     expected = {
         "type": "image_url",
@@ -1162,7 +1171,7 @@ def test_convert_to_openai_image_block() -> None:
         "source_type": "base64",
         "data": "<base64 data>",
         "mime_type": "image/jpeg",
-        "metadata": {"cache_control": {"type": "ephemeral"}},
+        "cache_control": {"type": "ephemeral"},
     }
     expected = {
         "type": "image_url",