fix(kg): sanitize LLM JSON output before parsing (#167)

Add sanitizeJSON() to fix malformed decimal numbers (e.g. '0. 85' → '0.85')
and trailing commas before closing brackets. Fixes extraction failures with
Gemini 2.5 Flash which occasionally produces invalid JSON.

internal/knowledgegraph/extractor_test.go

@@ -0,0 +1,163 @@
+package knowledgegraph
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestSanitizeJSON(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{
+			name:  "valid JSON unchanged",
+			input: `{"confidence": 0.85}`,
+			want:  `{"confidence": 0.85}`,
+		},
+		{
+			name:  "fix spaced decimal",
+			input: `{"confidence": 0. 85}`,
+			want:  `{"confidence": 0.85}`,
+		},
+		{
+			name:  "fix multiple spaced decimals",
+			input: `{"a": 0. 9, "b": 1. 0}`,
+			want:  `{"a": 0.9, "b": 1.0}`,
+		},
+		{
+			name:  "fix spaced decimal with multiple spaces",
+			input: `{"confidence": 0.   75}`,
+			want:  `{"confidence": 0.75}`,
+		},
+		{
+			name:  "preserve decimal-like pattern in strings",
+			input: `{"description": "Founded in 2023. 15 employees"}`,
+			want:  `{"description": "Founded in 2023. 15 employees"}`,
+		},
+		{
+			name:  "preserve spaced period in strings (no digit before dot)",
+			input: `{"description": "Mr. Smith leads the team"}`,
+			want:  `{"description": "Mr. Smith leads the team"}`,
+		},
+		{
+			name:  "mixed: fix value decimal, preserve string decimal",
+			input: `{"description": "Version 2. 0 alpha", "confidence": 0. 9}`,
+			want:  `{"description": "Version 2. 0 alpha", "confidence": 0.9}`,
+		},
+		{
+			name:  "fix trailing comma in array",
+			input: `{"items": [1, 2, 3,]}`,
+			want:  `{"items": [1, 2, 3]}`,
+		},
+		{
+			name:  "fix trailing comma in object",
+			input: `{"a": 1, "b": 2,}`,
+			want:  `{"a": 1, "b": 2}`,
+		},
+		{
+			name:  "trailing comma with whitespace",
+			input: `{"a": 1,  }`,
+			want:  `{"a": 1  }`,
+		},
+		{
+			name:  "trailing comma with newline",
+			input: "{\"a\": 1,\n}",
+			want:  "{\"a\": 1\n}",
+		},
+		{
+			name:  "preserve comma in string value",
+			input: `{"text": "hello, world,"}`,
+			want:  `{"text": "hello, world,"}`,
+		},
+		{
+			name:  "escaped quote in string",
+			input: `{"text": "she said \"0. 5\" loudly", "val": 0. 5}`,
+			want:  `{"text": "she said \"0. 5\" loudly", "val": 0.5}`,
+		},
+		{
+			name:  "nested structure",
+			input: `{"entities": [{"confidence": 0. 85,}]}`,
+			want:  `{"entities": [{"confidence": 0.85}]}`,
+		},
+		{
+			name:  "empty input",
+			input: "",
+			want:  "",
+		},
+		{
+			name:  "no fixes needed",
+			input: `{"entities": [], "relations": []}`,
+			want:  `{"entities": [], "relations": []}`,
+		},
+		{
+			name:  "truncated LLM response does not crash",
+			input: `{"entities": [{"name": "Facebook Ads", "confidence": 0.9}, {"name": "TikT`,
+			want:  `{"entities": [{"name": "Facebook Ads", "confidence": 0.9}, {"name": "TikT`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := sanitizeJSON(tt.input)
+			if got != tt.want {
+				t.Errorf("sanitizeJSON():\n  input: %s\n  got:   %s\n  want:  %s", tt.input, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestTruncatedJSONParseFails(t *testing.T) {
+	truncated := `{"entities": [{"name": "Facebook Ads", "confidence": 0.9}, {"name": "TikT`
+	sanitized := sanitizeJSON(truncated)
+
+	var result ExtractionResult
+	err := json.Unmarshal([]byte(sanitized), &result)
+	if err == nil {
+		t.Error("expected parse error for truncated JSON, got nil")
+	}
+}
+
+func TestStripCodeBlock(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{
+			name:  "no code block",
+			input: `{"entities": []}`,
+			want:  `{"entities": []}`,
+		},
+		{
+			name:  "json code block",
+			input: "```json\n{\"entities\": []}\n```",
+			want:  `{"entities": []}`,
+		},
+		{
+			name:  "plain code block",
+			input: "```\n{\"entities\": []}\n```",
+			want:  `{"entities": []}`,
+		},
+		{
+			name:  "code block with surrounding whitespace",
+			input: "  ```json\n{\"entities\": []}\n```  ",
+			want:  `{"entities": []}`,
+		},
+		{
+			name:  "empty input",
+			input: "",
+			want:  "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := stripCodeBlock(tt.input)
+			if got != tt.want {
+				t.Errorf("stripCodeBlock():\n  got:  %s\n  want: %s", got, tt.want)
+			}
+		})
+	}
+}