community: fix AzureSearch Oauth with azure_ad_access_token (#26995)

**Description:** 
AzureSearch vector store: create a wrapper class on
`azure.core.credentials.TokenCredential` (which is not-instantiable) to
fix Oauth usage with `azure_ad_access_token` argument

**Issue:** [the issue it
fixes](https://github.com/langchain-ai/langchain/issues/26216)

 **Dependencies:** None

- [x] **Lint and test**

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/community/langchain_community/vectorstores/azuresearch.py

@@ -119,6 +119,21 @@ def _get_search_client(
         VectorSearchProfile,
     )
 
+    class AzureBearerTokenCredential(TokenCredential):
+        def __init__(self, token: str):
+            # set the expiry to an hour from now.
+            self._token = AccessToken(token, int(time.time()) + 3600)
+
+        def get_token(
+            self,
+            *scopes: str,
+            claims: Optional[str] = None,
+            tenant_id: Optional[str] = None,
+            enable_cae: bool = False,
+            **kwargs: Any,
+        ) -> AccessToken:
+            return self._token
+
     additional_search_client_options = additional_search_client_options or {}
     default_fields = default_fields or []
     credential: Union[AzureKeyCredential, TokenCredential, InteractiveBrowserCredential]
@@ -131,11 +146,7 @@ def _get_search_client(
         else:
             credential = AzureKeyCredential(key)
     elif azure_ad_access_token is not None:
-        credential = TokenCredential(
-            lambda *scopes, **kwargs: AccessToken(
-                azure_ad_access_token, int(time.time()) + 3600
-            )
-        )
+        credential = AzureBearerTokenCredential(azure_ad_access_token)
     else:
         credential = DefaultAzureCredential()
     index_client: SearchIndexClient = SearchIndexClient(