avoid quadratic leaf re-adding in get_leaves_inside_matching_brackets

[sahvx655-wq]

Description

Profiling on inputs with deeply nested brackets kept pointing at
get_leaves_inside_matching_brackets in brackets.py, which runs while splitting
long lines (should_split_funcdef_with_rhs on return-type leaves, and the head
component of bracket_split_build_line). It gathers the ids of leaves sitting inside
matching bracket pairs, but on every matched closing bracket it re-adds the entire span
from the opening bracket to the closing one through an inner for j in range(start, i + 1)
loop. A leaf wrapped in k nested pairs is added k times, so the pass is O(n·depth)
and turns quadratic on something like a heavily subscripted annotation. A direct
microbenchmark shows the tell-tale 4x-per-doubling curve (a 4000-deep nested sequence
takes ~0.65s), and the helper is on the pre-auth formatting path reachable through
blackd, so a small crafted input buys disproportionate work.

Instead of re-adding the whole enclosed span each time a pair closes, each leaf id is
now collected under its innermost open bracket and flushed into the result only once,
when that bracket's matching close is reached; unmatched openings simply drop the ids
they collected. Keeping the bookkeeping on the bracket stack means every id is recorded
and unioned at most once, so the helper is linear while the returned set is identical. I
checked equivalence against the previous implementation over 200k randomised bracket
sequences and the full format suite stays byte-identical; the 4000-deep input drops from
~0.65s to ~0.005s.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy? (n/a, no style change)
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary? (byte-identical behaviour, covered by the existing format suite)
• Add new / update outdated documentation? (n/a)

[github-actions]

diff-shades results comparing this PR (0b00eef) to main (51abf53):

--preview style: no changes

--stable style: no changes

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