test(db): serialize postgres migration fixup tests

[serrrfirat]

Summary

• serialize the two PostgreSQL migration-fixup integration tests that both create/use the shared refinery_schema_history table
• avoids a CI race where concurrent CREATE TABLE IF NOT EXISTS refinery_schema_history can fail with a duplicate pg_type_typname_nsp_index error

Why

Main's Coverage (all-features) job failed on commit 576eb3bb7d6f01db6a92371243f838f059db11f1 in db::migration_fixup::tests::rejects_canonical_in_known_bad_checksums, not in the libSQL decimal parser test. The panic was:

duplicate key value violates unique constraint "pg_type_typname_nsp_index" ... refinery_schema_history

That points to an existing parallel Postgres DDL race, not a product-code regression.

Verification

• cargo fmt --check
• cargo test --lib --all-features db::migration_fixup::tests::rejects_canonical_in_known_bad_checksums -- --nocapture
• cargo check --all-features -q
• git diff --check

[gemini-code-assist]

Code Review

This pull request introduces a static mutex, POSTGRES_MIGRATION_FIXUP_TEST_LOCK, to synchronize integration tests within src/db/migration_fixup.rs. This change ensures that tests requiring the integration feature do not run concurrently, preventing potential race conditions during database migrations. I have no feedback to provide as there were no review comments.