fix: inject notebook name and description into chat system prompt

[lfnovo]

Summary

• Fetch the notebook linked to the chat session via the refers_to relationship in the /chat/execute endpoint
• Pass the notebook object into the LangGraph state so the Jinja2 template can render it
• Update the chat system prompt template to render notebook.name and notebook.description instead of the raw object

Closes #685

Test plan

• All 135 existing tests pass
• Open a notebook with a name and description, start a chat session, and verify the LLM response is contextually aware of the notebook topic

[cubic-dev-ai]

1 issue found across 2 files

Confidence score: 3/5

• There is a concrete user-impacting risk in api/routers/chat.py: when notebook lookup raises NotFoundError, it is mapped to "Session not found," which can fail valid sessions if the linked notebook is missing.
• I’m scoring this as moderate merge risk because the issue is medium severity (6/10) with high confidence (8/10) and affects request error handling behavior.
• Pay close attention to api/routers/chat.py - exception mapping may return the wrong failure mode and block otherwise valid session flows.

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="api/routers/chat.py">

<violation number="1" location="api/routers/chat.py:352">
P2: Notebook lookup can raise `NotFoundError` that is incorrectly converted to "Session not found", causing valid sessions to fail when linked notebook is missing.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P2: Notebook lookup can raise NotFoundError that is incorrectly converted to "Session not found", causing valid sessions to fail when linked notebook is missing.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At api/routers/chat.py, line 352:

<comment>Notebook lookup can raise `NotFoundError` that is incorrectly converted to "Session not found", causing valid sessions to fail when linked notebook is missing.</comment>

<file context>
@@ -342,6 +342,15 @@ async def execute_chat(request: ExecuteChatRequest):
+        )
+        notebook = None
+        if notebook_query:
+            notebook = await Notebook.get(notebook_query[0]["out"])
+
         # Determine model override (per-request override takes precedence over session-level)
</file context>