refactor(actor)!: Resolve and cache handlers by message type

[gfieni]

PR overview:

• Resolve handlers through the message class’s MRO, giving specific handlers precedence over catch-all handlers regardless of registration order.
• Cache successful resolutions by message class and clear the cache on every registration.
• Support later-loaded and deeper subclasses, and prevent collisions between unrelated message classes sharing a name.
• Move State related tests into test_state.py.

Breaking changes:

• Remove include_subclasses from State.add_handler(); inheritance matching is now unconditional.
• Use class objects instead of class names as keys in State.handlers.
• Rename State.dispatch_message() to State.handle_message().

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.77%. Comparing base (dbbd07d) to head (269fa3b).

Files with missing lines	Patch %	Lines
src/powerapi/actor/actor.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #953      +/-   ##
==========================================
+ Coverage   86.72%   86.77%   +0.04%     
==========================================
  Files          81       81              
  Lines        2817     2827      +10     
==========================================
+ Hits         2443     2453      +10     
  Misses        374      374              

Files with missing lines	Coverage Δ
src/powerapi/actor/handler.py	83.33% <100.00%> (ø)
src/powerapi/actor/state.py	92.10% <100.00%> (+2.81%)	⬆️
src/powerapi/actor/actor.py	75.82% <0.00%> (ø)

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