perf: find the versions directory without inspect.stack()

[jd]

get_down_revision(revision) locates the versions directory by walking
the call stack, and it did so with inspect.stack(), which also reads
the source context of every frame. It runs once per migration, so on a
long chain it dominated building the revision map: in the Mergify
engine's test suite, collecting the tree spent most of its time here,
because the collection stack is deep and 400 migration loads each paid
for it.

Walk the frame objects with f_back instead and read only
f_code.co_filename, which is all the lookup uses. Collecting the
engine's 15,680 tests goes from 20.8-31.8s to 6.2-9.0s. A new test
executes a real migration file from a versions/ directory, since the
existing one mocks the lookup out.

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 5.5 (1M context) noreply@anthropic.com

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What changed in this PR

Improves migration version-directory discovery performance by replacing inspect.stack() with direct frame traversal and adds real migration coverage.

Changes:

• Walks caller frames via f_back and co_filename.
• Cleans up frame references.
• Adds an integration-style migration test.

File	Description
tests/​test_chain.py	Tests discovery from an executed migration file.
alembic_git_revisions/​_chain.py	Optimizes caller-frame traversal.

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[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-09-23 17:56 UTC · Rule: default · triggered by merge protections
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-09-23 17:56 UTC · at 0da7d43fe15f490e73a7597cf155fa6455d95261 · squash

This pull request spent 10 seconds in the queue, including 2 seconds running CI.

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