fix: generateUpdateMask no period at root

[brianquinlan]

They key change is:

-    final nextPath = root.isEmpty ? '$root.$key' : '$key';
+    final nextPath = root.isEmpty ? key.toString() : '$root.$key';

I was changing the docs for this function at the code above just looked incorrect to me visually. But I don't know what this code is supposed to do so maybe I'm wrong ;-)

I've written unit tests to verify what I think that the code is supposed to do but someone who understands what the intent behind generateUpdateMask needs to verify.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #212   +/-   ##
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  Coverage        ?   74.59%           
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  Files           ?      105           
  Lines           ?     6724           
  Branches        ?        0           
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  Hits            ?     5016           
  Misses          ?     1708           
  Partials        ?        0           

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[gemini-code-assist]

Code Review

This pull request refactors the generateUpdateMask utility function to simplify its logic and improve path generation, while also adding comprehensive unit tests. The reviewer suggested an optimization to the recursive path collection to avoid redundant list iterations and string concatenations.

[gemini-code-assist]

The current implementation of _generateUpdateMask recursively calls itself and then manually prepends the current key to the results. This results in an O(N^2) string concatenation behavior due to repeated list iteration and string building. It is more efficient to pass the current path down the recursion and collect the final paths directly.

    final maskList = _generateUpdateMask(obj[key], nextPath);
    if (maskList.isNotEmpty) {
      updateMask.addAll(maskList);
    } else {
      updateMask.add(nextPath);
    }