fix(formatting): do not hide siblings of nested predefined fields

[pamburus]

When a predefined field is configured with a nested path (e.g., span.name), the nested field should be hidden from regular fields output without triggering the ... hidden fields indicator. However, the sibling fields (e.g., span.context) should not be hidden.

Changes:

• Add nested_field_names() method to PredefinedFields to iterate over nested predefined field paths
• Build a separate IncludeExcludeKeyFilter for predefined nested fields
• Add FormatResult enum to distinguish between formatted, hidden by user (shows ...), and hidden by predefined (silent skip)
• Pass predefined filter to formatter and check it before user filter
• Roll back empty objects that become empty due to predefined filtering

Additional fix for #176

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.28%. Comparing base (04abba1) to head (35112c9).
⚠️ Report is 77 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1187      +/-   ##
==========================================
+ Coverage   84.12%   84.28%   +0.16%     
==========================================
  Files          47       47              
  Lines        9465     9539      +74     
==========================================
+ Hits         7962     8040      +78     
+ Misses       1503     1499       -4     

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