Unpin langgraph stack; upgrade langchain-core for Reviver compat

Closes #10

Remove force-pins on langgraph-checkpoint/prebuilt/sdk and bump
langchain-core (and companions) so checkpoint 4.x Reviver(allowed_objects=...)
works. Keep langgraph on a supported range matching langchain 1.3.x.
Also switch BaseMessage .text() calls to the .text property.

pyproject.toml

@@ -18,19 +18,18 @@ dependencies = [
     "django-autoslug==1.9.9",
     "django-filter==25.2",
     "ollama==0.6.1",
-    "langchain==1.1.2",
-    "langchain-core==1.1.1",
-    "langchain-community==0.4.1",
-    "langchain-classic==1.0.0",
-    "langchain-ollama==1.0.0",
-    "langchain-chroma==1.0.0",
-    "langchain-text-splitters==1.0.0",
-    # Pin langgraph stack: newer langgraph-checkpoint calls
-    # Reviver(allowed_objects=...) which langchain-core 1.1.1 lacks.
-    "langgraph==1.0.4",
-    "langgraph-checkpoint==3.0.1",
-    "langgraph-prebuilt==1.0.5",
-    "langgraph-sdk==0.2.14",
+    "langchain==1.3.14",
+    "langchain-core==1.5.1",
+    "langchain-community==0.4.2",
+    "langchain-classic==1.0.8",
+    "langchain-ollama==1.1.0",
+    "langchain-chroma==1.1.0",
+    "langchain-text-splitters==1.1.2",
+    # langgraph stack is no longer force-pinned. langchain / langgraph
+    # require langchain-core>=1.4.7 (Reviver allowed_objects). Let
+    # langgraph resolve checkpoint / prebuilt / sdk transitively.
+    # Supported: langgraph>=1.2.5,<1.3 (matches langchain 1.3.x).
+    "langgraph>=1.2.5,<1.3.0",
     "chromadb==1.3.5",
     "unstructured==0.18.21",
     "pypdf==6.4.0",