Fix JSON breadcrumb filters for literal keys (#689)

packages/common-lib/jsonPath.ts

@@ -0,0 +1,39 @@
+/** Literal steps into a JSON response, distinct from JSONPath operators. */
+export type JsonPathSegment =
+  | { readonly kind: "key"; readonly key: string }
+  | { readonly kind: "index"; readonly index: number };
+
+const PATH_SEGMENT = /^(?:\.([A-Za-z_$][A-Za-z0-9_$]*)|\[(0|[1-9]\d*)\]|\[("(?:[^"\\]|\\.)*")\])/;
+
+/**
+ * Parse a single location with JSON-quoted keys (`$.a[0]["b.c"]`). Other
+ * JSONPath expressions belong to the filter engine, not to the breadcrumb trail.
+ */
+export function jsonPathToSegments(path: string): JsonPathSegment[] | null {
+  let rest = path.trim();
+  if (!rest.startsWith("$")) return null;
+  rest = rest.slice(1);
+
+  const segments: JsonPathSegment[] = [];
+  while (rest.length > 0) {
+    const match = PATH_SEGMENT.exec(rest);
+    if (match == null) return null;
+    const [whole, bareKey, index, quotedKey] = match;
+    if (bareKey != null) {
+      if (bareKey === "$") return null; // The library's root operator, not a literal key.
+      segments.push({ kind: "key", key: bareKey });
+    } else if (index != null) {
+      const value = Number(index);
+      if (!Number.isSafeInteger(value)) return null;
+      segments.push({ kind: "index", index: value });
+    } else if (quotedKey != null) {
+      try {
+        segments.push({ kind: "key", key: JSON.parse(quotedKey) as string });
+      } catch {
+        return null;
+      }
+    }
+    rest = rest.slice(whole.length);
+  }
+  return segments;
+}