fix(jsonpath): prevent panic on negative array index (#1703)

A jsonpath condition with a negative array index (e.g. [BODY].data[-1]
or [BODY][-1]) reached array[arrayIndex] with a negative index because
the len(array) > arrayIndex bounds checks are satisfied by any negative
value, causing a 'runtime error: index out of range [-1]' panic. Since
condition evaluation runs in the watchdog with no recover(), this would
crash the process.

Guard against negative indices alongside the existing strconv.Atoi error
check so an out-of-range negative index returns nil (treated as an
invalid path) instead of panicking. Adds tests for negative indices on
keyed arrays, root arrays, and negative-index-followed-by-key.

Made by an AI agent (Claude Code, model Claude Opus 4.8).

Co-authored-by: TwiN <twin@linux.com>

jsonpath/jsonpath.go

@@ -87,7 +87,7 @@ func extractValue(currentKey string, value interface{}) interface{} {
 			}
 		}
 		arrayIndex, err := strconv.Atoi(index)
-		if err != nil {
+		if err != nil || arrayIndex < 0 {
 			return nil
 		}
 		currentKeyWithoutIndex := currentKey[:startOfBracket]

jsonpath/jsonpath_test.go

@@ -174,6 +174,38 @@ func TestEval(t *testing.T) {
 			ExpectedOutputLength: 18,
 			ExpectedError:        false,
 		},
+		{
+			Name:                 "negative-index-on-keyed-array",
+			Path:                 "data[-1]",
+			Data:                 `{"data": [1, 2, 3]}`,
+			ExpectedOutput:       "",
+			ExpectedOutputLength: 0,
+			ExpectedError:        true,
+		},
+		{
+			Name:                 "negative-index-on-root-array",
+			Path:                 "[-1]",
+			Data:                 `[1, 2, 3]`,
+			ExpectedOutput:       "",
+			ExpectedOutputLength: 0,
+			ExpectedError:        true,
+		},
+		{
+			Name:                 "negative-index-followed-by-key",
+			Path:                 "data[-1].name",
+			Data:                 `{"data": [{"name": "value"}]}`,
+			ExpectedOutput:       "",
+			ExpectedOutputLength: 0,
+			ExpectedError:        true,
+		},
+		{
+			Name:                 "negative-index-nested-array",
+			Path:                 "data[0][-1]",
+			Data:                 `{"data": [[1, 2, 3]]}`,
+			ExpectedOutput:       "",
+			ExpectedOutputLength: 0,
+			ExpectedError:        true,
+		},
 	}
 	for _, scenario := range scenarios {
 		t.Run(scenario.Name, func(t *testing.T) {