fix(core): resolve cache validation error by safely converting Generation to ChatGeneration objects (#32156)

## Problem

ChatLiteLLM encounters a `ValidationError` when using cache on
subsequent calls, causing the following error:

```
ValidationError(model='ChatResult', errors=[{'loc': ('generations', 0, 'type'), 'msg': "unexpected value; permitted: 'ChatGeneration'", 'type': 'value_error.const', 'ctx': {'given': 'Generation', 'permitted': ('ChatGeneration',)}}])
```

This occurs because:
1. The cache stores `Generation` objects (with `type="Generation"`)
2. But `ChatResult` expects `ChatGeneration` objects (with
`type="ChatGeneration"` and a required `message` field)
3. When cached values are retrieved, validation fails due to the type
mismatch

## Solution

Added graceful handling in both sync (`_generate_with_cache`) and async
(`_agenerate_with_cache`) cache methods to:

1. **Detect** when cached values contain `Generation` objects instead of
expected `ChatGeneration` objects
2. **Convert** them to `ChatGeneration` objects by wrapping the text
content in an `AIMessage`
3. **Preserve** all original metadata (`generation_info`)
4. **Allow** `ChatResult` creation to succeed without validation errors

## Example

```python
# Before: This would fail with ValidationError
from langchain_community.chat_models import ChatLiteLLM
from langchain_community.cache import SQLiteCache
from langchain.globals import set_llm_cache

set_llm_cache(SQLiteCache(database_path="cache.db"))
llm = ChatLiteLLM(model_name="openai/gpt-4o", cache=True, temperature=0)

print(llm.predict("test"))  # Works fine (cache empty)
print(llm.predict("test"))  # Now works instead of ValidationError

# After: Seamlessly handles both Generation and ChatGeneration objects
```

## Changes

- **`libs/core/langchain_core/language_models/chat_models.py`**: 
  - Added `Generation` import from `langchain_core.outputs`
- Enhanced cache retrieval logic in `_generate_with_cache` and
`_agenerate_with_cache` methods
- Added conversion from `Generation` to `ChatGeneration` objects when
needed

-
**`libs/core/tests/unit_tests/language_models/chat_models/test_cache.py`**:
- Added test case to validate the conversion logic handles mixed object
types

## Impact

- **Backward Compatible**: Existing code continues to work unchanged
- **Minimal Change**: Only affects cache retrieval path, no API changes
- **Robust**: Handles both legacy cached `Generation` objects and new
`ChatGeneration` objects
- **Preserves Data**: All original content and metadata is maintained
during conversion

Fixes #22389.

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Co-authored-by: Mason Daugherty <mason@langchain.dev>
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libs/core/langchain_core/language_models/chat_models.py

@@ -11,22 +11,9 @@ from abc import ABC, abstractmethod
 from collections.abc import AsyncIterator, Iterator, Sequence
 from functools import cached_property
 from operator import itemgetter
-from typing import (
-    TYPE_CHECKING,
-    Any,
-    Callable,
-    Literal,
-    Optional,
-    Union,
-    cast,
-)
+from typing import TYPE_CHECKING, Any, Callable, Literal, Optional, Union, cast
 
-from pydantic import (
-    BaseModel,
-    ConfigDict,
-    Field,
-    model_validator,
-)
+from pydantic import BaseModel, ConfigDict, Field, model_validator
 from typing_extensions import override
 
 from langchain_core._api import deprecated
@@ -63,6 +50,7 @@ from langchain_core.outputs import (
     ChatGeneration,
     ChatGenerationChunk,
     ChatResult,
+    Generation,
     LLMResult,
     RunInfo,
 )
@@ -653,6 +641,34 @@ class BaseChatModel(BaseLanguageModel[BaseMessage], ABC):
     def _combine_llm_outputs(self, llm_outputs: list[Optional[dict]]) -> dict:  # noqa: ARG002
         return {}
 
+    def _convert_cached_generations(self, cache_val: list) -> list[ChatGeneration]:
+        """Convert cached Generation objects to ChatGeneration objects.
+
+        Handle case where cache contains Generation objects instead of
+        ChatGeneration objects. This can happen due to serialization/deserialization
+        issues or legacy cache data (see #22389).
+
+        Args:
+            cache_val: List of cached generation objects.
+
+        Returns:
+            List of ChatGeneration objects.
+        """
+        converted_generations = []
+        for gen in cache_val:
+            if isinstance(gen, Generation) and not isinstance(gen, ChatGeneration):
+                # Convert Generation to ChatGeneration by creating AIMessage
+                # from the text content
+                chat_gen = ChatGeneration(
+                    message=AIMessage(content=gen.text),
+                    generation_info=gen.generation_info,
+                )
+                converted_generations.append(chat_gen)
+            else:
+                # Already a ChatGeneration or other expected type
+                converted_generations.append(gen)
+        return converted_generations
+
     def _get_invocation_params(
         self,
         stop: Optional[list[str]] = None,
@@ -1010,7 +1026,8 @@ class BaseChatModel(BaseLanguageModel[BaseMessage], ABC):
                 prompt = dumps(messages)
                 cache_val = llm_cache.lookup(prompt, llm_string)
                 if isinstance(cache_val, list):
-                    return ChatResult(generations=cache_val)
+                    converted_generations = self._convert_cached_generations(cache_val)
+                    return ChatResult(generations=converted_generations)
             elif self.cache is None:
                 pass
             else:
@@ -1082,7 +1099,8 @@ class BaseChatModel(BaseLanguageModel[BaseMessage], ABC):
                 prompt = dumps(messages)
                 cache_val = await llm_cache.alookup(prompt, llm_string)
                 if isinstance(cache_val, list):
-                    return ChatResult(generations=cache_val)
+                    converted_generations = self._convert_cached_generations(cache_val)
+                    return ChatResult(generations=converted_generations)
             elif self.cache is None:
                 pass
             else: