Improve type annotations for several HTTPAdapter methods

[mrhx01]

improve type annotations for several HTTPAdapter methods
This PR improves the type hints for several methods in requests.adapters.HTTPAdapter to better reflect the actual runtime behavior and improve static type checking with tools such as mypy, pyright, and pylance.

The following methods now include more precise type annotations:

init_poolmanager
proxy_manager_for
add_headers
request_url
proxy_headers
These methods previously used untyped parameters or lacked explicit return types in the type stubs, which could lead to incomplete or inaccurate type inference in downstream projects.

Key improvements include:

Adding explicit parameter types (e.g. PreparedRequest, Mapping[str, str] | None)
Adding explicit return types such as None, str, and dict[str, str]
Using Any for flexible keyword arguments (**pool_kwargs, **proxy_kwargs, **kwargs) where the accepted parameters depend on urllib3 internals.
These changes do not modify runtime behavior and are strictly improvements to typing clarity and developer tooling support.

[srittau]

A general remark: We need a comment for each Any, explaining what types are allowed, why it's used instead of a concrete type, or how it's used.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[mrhx01]

I've addressed the review feedback: added explanatory comments for every Any usage in the stub file, including **pool_kwargs, **proxy_kwargs, return type of proxy_manager_for, **kwargs in add_headers, and the conn parameter in cert_verify. No functional changes, only type hint improvements.

Please let me know if anything else is needed.

A general remark: We need a comment for each Any, explaining what types are allowed, why it's used instead of a concrete type, or how it's used.

[srittau]

Thanks!