anthropic[patch]: always add tool_result type to ToolMessage content (#22721)

Anthropic tool results can contain image data, which are typically
represented with content blocks having `"type": "image"`. Currently,
these content blocks are passed as-is as human/user messages to
Anthropic, which raises BadRequestError as it expects a tool_result
block to follow a tool_use.

Here we update ChatAnthropic to nest the content blocks inside a
tool_result content block.

Example:
```python
import base64

import httpx
from langchain_anthropic import ChatAnthropic
from langchain_core.messages import AIMessage, HumanMessage, ToolMessage
from langchain_core.pydantic_v1 import BaseModel, Field


# Fetch image
image_url = "https://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Gfp-wisconsin-madison-the-nature-boardwalk.jpg/2560px-Gfp-wisconsin-madison-the-nature-boardwalk.jpg"
image_data = base64.b64encode(httpx.get(image_url).content).decode("utf-8")


class FetchImage(BaseModel):
    should_fetch: bool = Field(..., description="Whether an image is requested.")


llm = ChatAnthropic(model="claude-3-sonnet-20240229").bind_tools([FetchImage])

messages = [
    HumanMessage(content="Could you summon a beautiful image please?"),
    AIMessage(
        content=[
            {
                "type": "tool_use",
                "id": "toolu_01Rn6Qvj5m7955x9m9Pfxbcx",
                "name": "FetchImage",
                "input": {"should_fetch": True},
            },
        ],
        tool_calls=[
            {
                "name": "FetchImage",
                "args": {"should_fetch": True},
                "id": "toolu_01Rn6Qvj5m7955x9m9Pfxbcx",
            },
        ],
    ),
    ToolMessage(
        name="FetchImage",
        content=[
            {
                "type": "image",
                "source": {
                    "type": "base64",
                    "media_type": "image/jpeg",
                    "data": image_data,
                },
            },
        ],
        tool_call_id="toolu_01Rn6Qvj5m7955x9m9Pfxbcx",
    ),
]

llm.invoke(messages)
```

Trace:
https://smith.langchain.com/public/d27e4fc1-a96d-41e1-9f52-54f5004122db/r

libs/partners/anthropic/langchain_anthropic/chat_models.py

@@ -104,7 +104,12 @@ def _merge_messages(
     for curr in messages:
         curr = curr.copy(deep=True)
         if isinstance(curr, ToolMessage):
-            if isinstance(curr.content, str):
+            if isinstance(curr.content, list) and all(
+                isinstance(block, dict) and block.get("type") == "tool_result"
+                for block in curr.content
+            ):
+                curr = HumanMessage(curr.content)  # type: ignore[misc]
+            else:
                 curr = HumanMessage(  # type: ignore[misc]
                     [
                         {
@@ -114,8 +119,6 @@ def _merge_messages(
                         }
                     ]
                 )
-            else:
-                curr = HumanMessage(curr.content)  # type: ignore[misc]
         last = merged[-1] if merged else None
         if isinstance(last, HumanMessage) and isinstance(curr, HumanMessage):
             if isinstance(last.content, str):