Replace Primitives trait object by an enum

[wyfo]

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🏷️ Label-Based Checklist

Based on the labels applied to this PR, please complete these additional requirements:

Labels: internal

🏠 Internal Change

This PR is marked as internal (not user-facing):

• No API changes - Public APIs unchanged
• No behavior changes - External behavior identical
• Refactoring/maintenance - Code improvements only
• Tests still pass - All existing tests pass without modification

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[codecov]

Codecov Report

❌ Patch coverage is 92.95775% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.90%. Comparing base (3c77c34) to head (229ae7f).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
zenoh/src/net/runtime/mod.rs	92.06%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2375      +/-   ##
==========================================
- Coverage   71.91%   71.90%   -0.01%     
==========================================
  Files         390      390              
  Lines       62764    62829      +65     
==========================================
+ Hits        45139    45180      +41     
- Misses      17625    17649      +24     

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