[Refactor] Extract Complex Methods in TimeSeriesModel.fit_arch

[astrogilda]

Description

This PR addresses issue #182 by refactoring the complex TimeSeriesModel.fit_arch method into smaller, more manageable methods.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• All existing tests pass
• New unit tests added for extracted methods
• Manual testing of model creation

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

Additional Information

This refactoring improves code maintainability by:

• Breaking down a 95+ line method into focused, testable units
• Following the Single Responsibility Principle
• Making it easier to add new model types in the future

Resolves #182

[sonarqubecloud]

Please retry analysis of this Pull-Request directly on SonarQube Cloud