fix: handle GraphRecursionError as a graceful answer, not a 500

[devthedevil]

Problem

The supervisor↔worker loop is capped at RECURSION_LIMIT = 8 hops (agentstore/graph.py), which the README calls out as an intentional cost/latency bound. But if that budget is exhausted without the supervisor ever picking FINISH — a genuinely reachable case for a question spanning several domains, or if the LLM bounces indecisively between specialists — langgraph raises GraphRecursionError.

/api/chat's exception handling is a single blanket clause:

except Exception as exc:  # noqa: BLE001 — bubble up as 500 with a short msg
    raise HTTPException(status_code=500, detail=f"graph error: {exc}") from exc

So this expected, handled-in-principle outcome surfaces to the user as an opaque HTTP 500, with detail containing raw LangGraph internals, e.g.:

graph error: Recursion limit of 8 reached without hitting a stop condition. You can increase the limit by setting the recursion_limit config key. For troubleshooting, visit: https://docs.la...

The bundled frontend (frontend/index.html) renders any non-2xx response as a red error bubble, so a user who asks a legitimately complex question gets what looks like a broken app instead of a helpful nudge to rephrase.

I verified this is actually reachable (not just theoretical) by building a small graph with the same topology as graph.py (supervisor never returns FINISH) and confirming langgraph.errors.GraphRecursionError — not some other exception — is what gets raised at the configured limit.

Fix

Catch GraphRecursionError specifically in the /api/chat handler and return it as a normal ChatResponse (200) with a clear, actionable message, instead of falling through to the generic 500 handler:

except GraphRecursionError:
    return ChatResponse(
        answer=RECURSION_LIMIT_MESSAGE,
        agent="supervisor",
        latency_ms=int((time.perf_counter() - start) * 1000),
    )

This keeps the existing except Exception clause as the true "something actually broke" path, and gives this specific, expected outcome its own graceful handling — matching the ChatResponse contract the frontend already knows how to render as a normal chat bubble.

Testing

• Added test_chat_handles_recursion_limit_gracefully, which stubs the graph to raise GraphRecursionError and asserts a 200 with the friendly message (mirrors the existing stub-graph pattern already used in this test file).
• Full suite: ruff check clean, pytest 24/24 passing locally.