feat: only construct handlers once

[bdraco]

When bleak is using passive scanning each message would cause a method handler to be constructed via _make_method_handler

Only construct them once.

[codecov]

Codecov Report

Base: 82.11% // Head: 82.15% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (de38a00) compared to base (2fa7cdb).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
+ Coverage   82.11%   82.15%   +0.03%     
==========================================
  Files          27       27              
  Lines        3137     3144       +7     
  Branches      647      649       +2     
==========================================
+ Hits         2576     2583       +7     
  Misses        343      343              
  Partials      218      218              

Impacted Files	Coverage Δ
src/dbus_fast/message_bus.py	71.27% <100.00%> (ø)
src/dbus_fast/service.py	88.58% <100.00%> (+0.28%)	⬆️

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[bdraco]

Tested in production with passive scanning.

Much better.

There are still parts that search the whole message for fds and will never do anything.