[UI] Enable strict checking for service contracts and route handler

[PunugupatiSaikumar]

Changes

• Enable strict checking for AbstractService and GlobalRouteChangeHandler.
• Add explicit service return types.
• Match DummyService.toast to the Promise contract without changing its throwing behavior.
• Type nullable route snapshots and metadata using Angular types.

No runtime logic changes or new test files.

Related to #3995

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #4047      +/-   ##
=============================================
- Coverage      55.53%   55.45%   -0.07%     
- Complexity     19904    19907       +3     
=============================================
  Files           3916     3931      +15     
  Lines         180171   180759     +588     
  Branches       15034    15077      +43     
=============================================
+ Hits          100037   100221     +184     
- Misses         76150    76564     +414     
+ Partials        3984     3974      -10     

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