Refactor handler matching and validate command entity bounds

[negasus]

This replaces the complex handler matching function with separate update extraction and matching helpers, incorporating the refactoring and tests from #222. Invalid command entities are skipped without panics, integer overflow, or false matches against an empty pattern. Missing update data uses a success flag instead of allocating an error.

The public API and existing matching behavior for valid input remain unchanged. The existing UTF-16 entity-offset issue is left for a separate fix. Credit for the original refactoring is preserved in the commit's co-author trailer.

Validation:

• Tests passed for all packages and examples, plus race checks for the bot and models packages.
• Handler statement coverage remains 100%.
• Comparison against the previous implementation passed for 2,625 input combinations.
• New lint findings: none. Full local lint reports 16 findings in unchanged files.
• Skipping handlers with missing update data allocates zero bytes.

Supersedes #222.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.45%. Comparing base (6fd12c4) to head (36b648f).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #304      +/-   ##
==========================================
+ Coverage   55.81%   56.45%   +0.64%     
==========================================
  Files          34       34              
  Lines        2917     2942      +25     
==========================================
+ Hits         1628     1661      +33     
+ Misses       1240     1236       -4     
+ Partials       49       45       -4     

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