chore(typing): modernizes codebase to use Python 3.9+ built-in types

[mvilanova]

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[Copilot]

Pull Request Overview

This PR modernizes the codebase to leverage built‐in type annotations and generic types introduced in recent Python versions. Key changes include replacing typing.Optional and typing.List with the union operator (|) and built‐in generics (e.g. list[..]), and updating function signatures in service, model, view, and helper modules accordingly.

Reviewed Changes

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File	Description
src/dispatch/data/alert/service.py	Updated return types from Optional to Alert
src/dispatch/data/alert/models.py	Converted Optional and List types to union types and built‐in generics
src/dispatch/cost_model/service.py	Changed List returns to list[...] using the new built‐in generics
src/dispatch/conversation/service.py	Updated function return annotations from Optional to union type
src/dispatch/config.py	Converted typing.List parameter in get_env_tags to list[...]
src/dispatch/conference/service.py	Updated return types from Optional to union types
src/dispatch/conference/flows.py	Modernized list parameters in function signatures
src/dispatch/case_cost_type/service.py	Updated function return types from Optional to union types
src/dispatch/case_cost_type/models.py	Replaced Optional types with union types for better consistency
src/dispatch/case_cost/service.py	Updated various return types from List[Optional[...]] to list[...]
src/dispatch/case_cost/models.py	Converted Optional and List annotations to union and built‐in generics
src/dispatch/case/views.py	Changed response_model annotations to use built‐in list types
src/dispatch/case/type/service.py	Replaced Optional with union types in function signatures
src/dispatch/case/severity/service.py	Updated return types to use union types
src/dispatch/case/service.py	Modernized nearly all Optional and List type hints in service functions
src/dispatch/case/priority/service.py	Updated function annotations with the new union operator