Annotate the serializable decorators and fix the error type of Result

[renan-r-santos]

This PR fixes two typing gaps that make serialite consumers lose type information. There is no runtime behavior change.

First, serializable and abstract_serializable had no annotations. Type checkers therefore treat every decorated class as passing through an untyped decorator and degrade or flag it. Both decorators now declare the identity signature (cls: type[T]) -> type[T], which is what checkers already assume. flexible_decorator is typed as (dec: F) -> F and casts its wrapper, so the inner signature is preserved through it.

Second, in _result.py the annotated alias Failure: type[result.Failure[Errors]] was immediately overwritten by Failure = result.Failure, so the Errors parameterization was lost. As a result, Result[T].failure() evaluated to Any instead of Errors. The plain class assignment is required because class patterns like case Failure(...) need the actual class at runtime. The fix keeps the plain re-export and moves the parameterization into the alias.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.70%. Comparing base (3aad53f) to head (7d8bee6).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #59      +/-   ##
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- Coverage   92.71%   92.70%   -0.01%     
==========================================
  Files          35       35              
  Lines        1564     1563       -1     
  Branches      236      236              
==========================================
- Hits         1450     1449       -1     
  Misses         49       49              
  Partials       65       65              

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