feat(api): Ollama /api/chat client with one retry + typed ModelError

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

api/app/ollama_client.py

@@ -0,0 +1,46 @@
+"""Call Ollama's /api/chat (non-streaming) with a single retry. Callers see
+success or ModelError (charter §12/§13 — one retry, then fail; the client
+degrades on any non-200). Built so a streaming mode is an additive path later.
+"""
+
+import httpx
+
+from . import config
+
+
+class ModelError(Exception):
+    """The model call failed after one retry (transport / timeout / non-2xx / empty)."""
+
+
+def chat(
+    model: str,
+    messages: list[dict],
+    options: dict,
+    *,
+    think: bool | None = None,
+    client: httpx.Client | None = None,
+) -> str:
+    payload: dict = {"model": model, "messages": messages, "stream": False, "options": options}
+    if think is not None:
+        payload["think"] = think
+
+    owns = client is None
+    http = client or httpx.Client(
+        base_url=config.ollama_base_url(), timeout=config.ollama_timeout_seconds()
+    )
+    try:
+        last = "no attempt made"
+        for _ in range(2):  # one try + one retry (§12: never more than once)
+            try:
+                resp = http.post("/api/chat", json=payload)
+                resp.raise_for_status()
+                content = resp.json().get("message", {}).get("content", "")
+                if content.strip():
+                    return content
+                last = "empty response from model"
+            except httpx.HTTPError as exc:  # TimeoutException, TransportError, HTTPStatusError
+                last = f"{type(exc).__name__}: {exc}"
+        raise ModelError(last)
+    finally:
+        if owns:
+            http.close()