Initialize domain before starting engine

[subhashb]

This ensures that all domain elements are registered and all adapters are initialized before the server starts processing messages.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.34%. Comparing base (97a2dde) to head (cd1cd2d).

Files	Patch %	Lines
src/protean/server/engine.py	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
+ Coverage   89.81%   90.34%   +0.53%     
==========================================
  Files          71       71              
  Lines        6000     6009       +9     
  Branches     1203     1205       +2     
==========================================
+ Hits         5389     5429      +40     
+ Misses        458      425      -33     
- Partials      153      155       +2     

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