test: add PostgreSQL to migration CI tests (#12257)

* test: add PostgreSQL to migration CI tests

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* use random pg name

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* fix: rewrite _get_main_branch_head to use git grep instead of filename parsing

The old approach used --diff-filter=A and extracted revision IDs from
filenames, which broke in two ways: filenames don't always match actual
revision IDs, and modified-only migrations were silently treated as a
no-op. Now uses git grep to read revision/down_revision directly from
origin/main's migration files and computes the head from the chain.

Also skips test_upgrade_from_main_branch with a clear message when main
and branch share the same head, instead of silently doing nothing.

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* ruff

* Add postgres to install

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* comp index

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Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>