style(types): fix return type annotation for get_all_requests

[xiaoajie738]

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Motivation

question1: The function actually returns a dictionary where each RequestType
maps to a list of Request objects. This change updates the type
hint to Dict[RequestType, List[Request]] for correctness and better
IDE/static type support.
question2: Previously, RequestManager.step accessed reqs_by_type[req_type] directly,
and the check if req_type not in reqs_by_type or len(reqs_by_type) == 0
was logically incorrect. This could cause the empty request queue to keep executing, wasting resources.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has a dependency on downstream projects of a newer version, this PR should be tested with all supported versions of downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[grimoire]

LGTM