refactor!: Rename MultichainRouter to MultichainRoutingService and use messenger exposed methods pattern

[Mrtenz]

This updates the MultichainRouter to use messenger exposed methods. Actions were already named properly.

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Note

Medium Risk
Breaking rename and action-type reshaping will require downstream updates, and the switch to SnapController:getRunnableSnaps changes the dependency contract for protocol snap routing. Runtime logic appears largely unchanged but touches request routing paths.

Overview
Renames the multichain RPC router from MultichainRouter to MultichainRoutingService and updates exported types/namespace action strings accordingly (breaking change).

Refactors messenger integration to use registerMethodActionHandlers with auto-generated method action types, and changes protocol-snap discovery to call SnapController:getRunnableSnaps directly (no longer requires SnapController:getAllSnaps). Tests and controller test-utils are updated to use the new messenger helpers and action names, and the changelog documents both breaking changes.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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##             controller-refactors    #3913   +/-   ##
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  Lines                           ?    12335           
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  Hits                            ?    12157           
  Misses                          ?      178           
  Partials                        ?        0           

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