Simplify BracketTracker initialization

[Faraax66]

Removing the early default value for bracket_depth since it's always set before use in practice. Should reduce redundant assignment.

[Faraax66]

⚠️ Heads-up: this area has a recent history of crashes caused by bracket_depth not being initialized before use. PR psf#5161 fixed an AttributeError: 'Leaf' object has no attribute 'bracket_depth' crash on valid code, and PR psf#5215 was a follow-up that explicitly ensured Leaf.bracket_depth is always initialized. The assumption that bracket_depth is "always set before use in practice" is what led to those bugs — please make sure to audit all code paths carefully, including edge cases like # fmt: skip after opening brackets, before removing this default initialization.

[msftcangoblowm]

I'm the author of black#5214

I agree with your main point.

You mentioned # fmt: skip, but which tags make up these long lines does not contribute to the crash.

This particular issue is triggered by:

1. 1st long line (triggers black.linegen.transform_line)
2. 2nd long line black struggles whether or not to enclose it with visible paranthesis. One inline comment ... crash Two inline comments ... different issue for another day

Can exchange # fmt: skip for say: # isort:skip and the crash would still occur.

Lets keep it simple and just wait for PR black#5215 to be merged.