Type hint: Add hints to TestMessageBox in ui_tools/test_messages.py

[kushalnsetty]

What does this PR do, and why?

This PR adds comprehensive type hints to the TestMessageBox class and its associated helper functions within zulipterminal/ui_tools/test_messages.py.

As part of the effort to improve the project's type safety, this change:

• Provides explicit types for parameters in various @pytest.mark.parametrize decorators.
• Type-hints fixtures such as mocker, message_fixture, and widget_size.
• Enables mypy for this file by removing the corresponding exclusion from pyproject.toml.

External discussion & connections

• Fully fixes Add type hints to test_messages.py #1337

How did you test this?

• Existing automated tests should already cover this (only a refactor of tested code)
• Verified type-checking by running mypy zulipterminal/ui_tools/messages.py.
• Verified test integrity by running pytest zulipterminal/ui_tools/messages.py.

Self-review checklist for each commit

• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It flows clearly from a previous branch commit, and/or prepares for the next commit

[kushalnsetty]

I've rebased the branch to include the issue reference #1337 in the commit messages as requested. Thanks for the guidance!

[kushalnsetty]

@neiljp i have opened this PR for #1337 please Review and let me know of any issues.
Thank you

[Niloth-p]

Please fix the failing tests.

[kushalnsetty]

@Niloth-p thankyou for your insights
8 out of 11 tests are passed and i dont know why the remaining tests are failing and how to fix it can you please guide me through it

and my apology for replying so lately