Bump Modal sandbox extra to 1.4.3

[jdoughty04]

Summary

• Bumps the optional Modal sandbox extra from modal==1.3.5 to modal==1.4.3.
• Refreshes uv.lock, including Modal's updated synchronicity dependency and removed transitive typer dependency.
• Unblocks Modal client APIs added in 1.4.3, including the newer sandbox filesystem methods requested in Is it possible to bump the Modal version? #3529.

Test plan

• uv lock --check
• uv run --extra modal python -c "import modal; print(modal.__version__ if hasattr(modal, '__version__') else 'modal import ok')" -> 1.4.3
• uv run --extra modal pytest -q tests/extensions/sandbox/test_modal.py -> 75 passed
• uv run ruff format --check
• uv run ruff check
• git diff --check

make is not installed in my Windows environment, so I ran the equivalent uv run ruff ... commands from the Makefile directly.

Issue number

Closes #3529

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass