fix(core): correct return type hints in BaseChatPromptTemplate (#32009)

This PR changes the return type hints of the `format_prompt` and
`aformat_prompt` methods in `BaseChatPromptTemplate` from `PromptValue`
to `ChatPromptValue`. Since both methods always return a
`ChatPromptValue`.

Author: AndreasVJ

libs/core/langchain_core/prompts/chat.py

@@ -35,7 +35,7 @@
     convert_to_messages,
 )
 from langchain_core.messages.base import get_msg_title_repr
-from langchain_core.prompt_values import ChatPromptValue, ImageURL, PromptValue
+from langchain_core.prompt_values import ChatPromptValue, ImageURL
 from langchain_core.prompts.base import BasePromptTemplate
 from langchain_core.prompts.dict import DictPromptTemplate
 from langchain_core.prompts.image import ImagePromptTemplate
@@ -715,20 +715,20 @@ async def aformat(self, **kwargs: Any) -> str:
         """
         return (await self.aformat_prompt(**kwargs)).to_string()
 
-    def format_prompt(self, **kwargs: Any) -> PromptValue:
-        """Format prompt. Should return a PromptValue.
+    def format_prompt(self, **kwargs: Any) -> ChatPromptValue:
+        """Format prompt. Should return a ChatPromptValue.
 
         Args:
             **kwargs: Keyword arguments to use for formatting.
 
         Returns:
-            PromptValue.
+            ChatPromptValue.
         """
         messages = self.format_messages(**kwargs)
         return ChatPromptValue(messages=messages)
 
-    async def aformat_prompt(self, **kwargs: Any) -> PromptValue:
-        """Async format prompt. Should return a PromptValue.
+    async def aformat_prompt(self, **kwargs: Any) -> ChatPromptValue:
+        """Async format prompt. Should return a ChatPromptValue.
 
         Args:
             **kwargs: Keyword arguments to use for formatting.