json_patch: fix move/copy false rejection between sibling keys sharing a prefix

json_patch_apply_move_copy() checks that "from" is not a proper prefix
of "path" per RFC 6902 section 4.4 (a location cannot be moved into one
of its own children), but it did so with a plain strncmp() over the raw
pointer strings, without checking that the shared run of characters
actually ends on a JSON Pointer reference-token boundary in "path".

As a result, moving or copying between two sibling object keys where
one name is a textual prefix of the other, e.g. "/aa" to "/aab", was
rejected with "Invalid attempt to move parent under a child" even
though "/aa" is not an ancestor pointer of "/aab" at all, just an
unrelated key that happens to share a few leading characters.

The fix requires that the character in "path" right after the shared
"from" prefix be either the end of the string (equal paths, a no-op)
or a '/' (a genuine nested reference token), before treating it as an
attempt to move a location under its own descendant.

Added two cases to json_patch_tests.json covering the move and copy
operations between such sibling keys, and refreshed the corresponding
golden test_json_patch.expected output.

tests/json_patch_tests.json

@@ -370,6 +370,16 @@
       "patch": [{"op": "move", "from": "/foo", "path": "/foo"}],
       "expected": {"foo": 1} },
 
+    { "comment": "Move between sibling keys sharing a name prefix is not a parent/child move",
+      "doc": {"aa": 1, "aab": 2},
+      "patch": [{"op": "move", "from": "/aa", "path": "/aab"}],
+      "expected": {"aab": 1} },
+
+    { "comment": "Copy between sibling keys sharing a name prefix is not a parent/child move",
+      "doc": {"aa": 1, "aab": 2},
+      "patch": [{"op": "copy", "from": "/aa", "path": "/aab"}],
+      "expected": {"aa": 1, "aab": 1} },
+
     { "doc": {"foo": 1, "baz": [{"qux": "hello"}]},
       "patch": [{"op": "move", "from": "/foo", "path": "/bar"}],
       "expected": {"baz": [{"qux": "hello"}], "bar": 1} },