fix: Update prompt message content types to use Literal and add union type for content (#17136)

Co-authored-by: 朱庆超 <zhuqingchao@xiaomi.com>
Co-authored-by: crazywoola <427733928@qq.com>

api/tests/unit_tests/core/prompt/test_prompt_message.py

@@ -0,0 +1,27 @@
+from core.model_runtime.entities.message_entities import (
+    ImagePromptMessageContent,
+    TextPromptMessageContent,
+    UserPromptMessage,
+)
+
+
+def test_build_prompt_message_with_prompt_message_contents():
+    prompt = UserPromptMessage(content=[TextPromptMessageContent(data="Hello, World!")])
+    assert isinstance(prompt.content, list)
+    assert isinstance(prompt.content[0], TextPromptMessageContent)
+    assert prompt.content[0].data == "Hello, World!"
+
+
+def test_dump_prompt_message():
+    example_url = "https://example.com/image.jpg"
+    prompt = UserPromptMessage(
+        content=[
+            ImagePromptMessageContent(
+                url=example_url,
+                format="jpeg",
+                mime_type="image/jpeg",
+            )
+        ]
+    )
+    data = prompt.model_dump()
+    assert data["content"][0].get("url") == example_url