Initialize Leaf bracket depth

[michaelsam94]

Description

Fixes #5214.

Leaf.bracket_depth is annotated and later read by formatting code, but a newly created leaf did not initialize the instance attribute until bracket tracking visited it. This initializes new leaves with depth 0, matching the default pre-bracket-tracking state, and adds a small regression test covering the leaf metadata defaults.

Checklist

• Implement any code style changes under --preview style, following stability policy?
• Add entry in CHANGES.md necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

No style or documentation changes are needed for this crash fix.

Tests

• .venv/bin/python -m pytest tests/test_pytree.py
• .venv/bin/python -m pytest tests/test_format.py -k fmtskip_after_bracket_with_comment
• .venv/bin/python -m black --check src/blib2to3/pytree.py tests/test_pytree.py

[msftcangoblowm]

Thank you for addressing this issue so quickly. Not in my wildest dreams did i expect that kinda turnaround.

This PR is sufficient to resolve this particular crash.

But this entire class of issues can all be resolved! 100s or 1000s of them.

• Proper (authoritative) stubs (not just typing within code modules)
• stubtest
• static typing (not runtime) tests using pytest-mypy-plugins

For example, before assert leaf.bracket_depth == 0, a runtime value check, should be a static typing check:

from typing_extensions import assert_type
assert_type(leaf.bracket_depth, int)

Then continue for the entire Leaf interface, not just the one attribute.
This static typing tests live within pytest-mypy-plugins yml files.

Of course, rtfm pytest-mypy-plugins. Can then visit types-strictyaml to see more realistic static typing tests.

[cobaltt7]

Thanks! One nit: could you put the test in an existing file instead of making a new one? Please also add a python test case file that provides an example of code that crashes without this fix.

[github-actions]

diff-shades results comparing this PR (f651f39) to main (c4c9a93):

--preview style: no changes

--stable style: no changes

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