fix(skills): tolerate a stray brace before the JSON in skill extraction (#2200)

maybe_extract_skill() sliced the LLM response from the first '{' to the
last '}'. When a model emits a stray brace in prose before the real
object (e.g. "uses {placeholder} then {...}"), the slice starts at the
prose brace, json.loads fails, and a valid skill is silently dropped.

Factor parsing into _extract_json_object(), which tries the whole
(de-fenced) string first and then each '{' start position, returning the
first candidate that parses to a JSON object.

Adds tests/test_skill_extractor_json.py.

tests/test_skill_extractor_json.py

@@ -0,0 +1,43 @@
+"""Regression: skill-extraction JSON parsing must tolerate a stray brace in prose.
+
+maybe_extract_skill() sliced the LLM response from the first '{' to the last
+'}'. When a model emits a stray brace in prose before the real object
+(e.g. "uses {placeholder} then {...}"), that slice starts at the prose brace and
+json.loads fails, so a perfectly good skill is silently dropped. Extraction now
+tries each '{' start position and returns the first candidate that parses to a
+JSON object.
+"""
+from services.memory import skill_extractor
+
+
+def test_stray_brace_before_real_json_is_recovered():
+    resp = (
+        'The user mentioned {placeholder} before the actual JSON '
+        '{"title": "Restart the service", "steps": ["a", "b"]}'
+    )
+    data = skill_extractor._extract_json_object(resp)
+    assert isinstance(data, dict)
+    assert data["title"] == "Restart the service"
+
+
+def test_clean_json_object():
+    data = skill_extractor._extract_json_object('{"title": "Y", "steps": []}')
+    assert data["title"] == "Y"
+
+
+def test_code_fenced_json():
+    data = skill_extractor._extract_json_object('```json\n{"title": "Z"}\n```')
+    assert data["title"] == "Z"
+
+
+def test_no_json_object_returns_none():
+    assert skill_extractor._extract_json_object("just prose, no object here") is None
+
+
+def test_non_object_json_returns_none():
+    # A bare array is valid JSON but not a skill object.
+    assert skill_extractor._extract_json_object("[1, 2, 3]") is None
+
+
+def test_empty_input_returns_none():
+    assert skill_extractor._extract_json_object("") is None