#1576

[Moksha25-tech]

What does this PR do, and why?

This PR implements TypedDict for the Submessage structure and centralizes it in api_types.py.

Key Motivations:

1. Type Safety: Moving away from generic Dict[str, Any] to TypedDict allows mypy to catch errors where submessage keys (like id or msg_type) might be missing or mistyped.
2. Consistency: Standardizes the field names across the app. We moved from the inconsistent use of submessage_id and type to a unified id and msg_type structure.
3. Centralization: Following Zulip Terminal patterns, all API-related structures are now in api_types.py for easier reuse.

Files Changed:

1. zulipterminal/api_types.py: Added Submessage, TodoWidgetResult, and PollWidgetResult TypedDicts.
2. zulipterminal/model.py: Updated submessage handling logic to use new keys and imports.
3. zulipterminal/widget.py: Refactored widget processing to return the new TypedDicts.
4. tests/conftest.py: Added submessage_factory to simplify and type-check test data creation.
5. tests/widget/test_widget.py & tests/model/test_model.py: Updated imports and test expectations to match the new structure.
6. tests/helper/test_helper.py & tests/cli/test_run.py: Fixed pre-existing environment-related failures (path normalization and CLI output capturing) to ensure a green test suite in WSL/Linux.

Outstanding aspect(s)

• Currently, mypy is run with --follow-imports=skip' to avoid syntax errors in third-party libraries (like packaging`) found in the local virtual environment.

External discussion & connections

• Fully fixes Improve typing & test approach for widgets #1576

How did you test this?

• Manually - Behavioral changes
• Adapting existing automated tests
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior

[Moksha25-tech]

@neiljp @zulipbot I wanted to request review for my PR as I am done working on the issue from my end. Thanks!