[Refactor] Extract Complex Methods in TimeSeriesModel.fit_arch

[astrogilda]

Is your feature request related to a problem? Please describe.
The TimeSeriesModel.fit_arch method is currently 95+ lines long with multiple conditional branches, making it difficult to understand, test, and maintain.

Describe the solution you'd like
Extract the model creation logic into separate factory methods:

• Extract GARCH model creation into _create_garch_model()
• Extract EGARCH model creation into _create_egarch_model()
• Extract TGARCH model creation into _create_tgarch_model()
• Extract common parameter validation into _validate_arch_params()

Describe alternatives you've considered

• Strategy pattern for model creation
• Builder pattern for complex model configurations

Additional context
This refactoring will:

• Improve code readability
• Enable unit testing of individual model creation logic
• Make it easier to add new model types in the future
• Follow the Single Responsibility Principle

Part of codebase quality improvement initiative identified in refactoring analysis.