Harden review JSON parsing against reasoning models

Some Fireworks models emit chain-of-thought before the JSON review, and
that preamble can contain stray "{". The old first-"{"-to-last-"}" span
then grabbed reasoning braces and failed to parse (seen live on a real
PR). Scan each "{" and return the first substring that actually decodes
to an object instead, so a preamble or trailing prose no longer breaks
the review.

app/reviewer.py

@@ -173,7 +173,18 @@ def _safe_json(text: str) -> dict[str, Any]:
     try:
         return json.loads(text)
     except json.JSONDecodeError:
-        start, end = text.find("{"), text.rfind("}")
-        if start != -1 and end != -1:
-            return json.loads(text[start : end + 1])
-        raise
+        pass
+    # Reasoning models often emit chain-of-thought (which may contain stray "{")
+    # before the JSON object. Scan every "{" and return the first that decodes to
+    # an object, rather than assuming the span from the first "{" to the last "}".
+    decoder = json.JSONDecoder()
+    for i, ch in enumerate(text):
+        if ch != "{":
+            continue
+        try:
+            obj, _ = decoder.raw_decode(text[i:])
+        except json.JSONDecodeError:
+            continue
+        if isinstance(obj, dict):
+            return obj
+    raise json.JSONDecodeError("no JSON object found", text, 0)

tests/test_reviewer.py

@@ -36,6 +36,12 @@ def test_safe_json_prose_wrapped() -> None:
     assert _safe_json(text) == {"summary": "ok"}
 
 
+def test_safe_json_skips_reasoning_braces() -> None:
+    # A reasoning preamble containing stray "{" must not confuse extraction.
+    text = 'We should return {the result}. Final: {"summary": "ok", "block": []} done.'
+    assert _safe_json(text) == {"summary": "ok", "block": []}
+
+
 def test_safe_json_malformed_raises() -> None:
     with pytest.raises(json.JSONDecodeError):
         _safe_json("this is not json at all")