refactor: mypy disallow_any_generics

[peterschutt]

This issue was identified in #2689 when we realized that mypy is inferring Litestar.template_engine as Any due to AppConfig.template_config being annotated as TemplateConfig (without generic arg) instead of TemplateConfig[TemplateEngineProtocol].

AppConfig.template_config is annotated here:

litestar/litestar/config/app.py

Line 199 in 38f3b19

template_config: TemplateConfig | None = field(default=None)

And assigned to the app instance here:

litestar/litestar/app.py

Line 412 in 38f3b19

self.template_engine = config.template_config.engine_instance if config.template_config else None

I really think we should work toward changing

litestar/pyproject.toml

Line 225 in 38f3b19

disallow_any_generics = false

to true so these types of errors cannot occur.

Originally posted by @peterschutt in #2689 (comment)

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