Fixed mixins.startup.serve UnboundLocalError.

[pygeek]

Fixes #2985

the finally block in mixins.serve references a variable that is only defined in the try block. This results in a UnboundLocalError within the finally block.

[pygeek]

There do not seem to be existing tests that specifically test the functionality of mixins.startup.serve, nor the startup module overall.

[pygeek]

Type error being raised by CI by line 724 of mixins.startup existed prior to my changes. https://github.com/sanic-org/sanic/actions/runs/10174510874/job/28140377393?pr=2992#step:2:566

Same for the 8 other mypy type-check errors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.451%. Comparing base (fcbd747) to head (0d7b2ae).
Report is 32 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2992       +/-   ##
=============================================
- Coverage   87.617%   87.451%   -0.167%     
=============================================
  Files          101       101               
  Lines         7575      7666       +91     
  Branches      1329      1350       +21     
=============================================
+ Hits          6637      6704       +67     
- Misses         641       661       +20     
- Partials       297       301        +4     

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