feat: auto-derive naming rules from MetaData.naming_convention

[AlexeyShalaev]

Summary

• test_naming_conventions now reads orm_metadata.naming_convention (already a required fixture) and auto-derives prefix/suffix rules from it
• Rules are merged in three layers — built-in defaults → convention → explicit class attributes (last wins)
• The SQLAlchemy built-in default {"ix": "ix_%(column_0_label)s"} is ignored so projects without a custom convention keep existing behaviour
• New alembic_gauntlet/utils/convention.py with rules_from_metadata() and NamingConventionRules

Zero-config usage

DB_NAMING_CONVENTION = {
    "ix": "%(column_0_label)s_idx",
    "uq": "%(table_name)s_%(column_0_name)s_key",
    "ck": "%(table_name)s_%(constraint_name)s_check",
    "fk": "%(table_name)s_%(column_0_name)s_fkey",
    "pk": "%(table_name)s_pkey",
}
Base = declarative_base(metadata=MetaData(naming_convention=DB_NAMING_CONVENTION))

class TestMyMigrations(MigrationTestBase):
    @pytest.fixture
    def orm_metadata(self) -> MetaData:
        return Base.metadata  # naming rules derived automatically — no class attributes needed

Test plan

• Existing integration test passes (no naming_convention → falls back to defaults)
• Unit tests for _extract_prefix, _extract_suffix, rules_from_metadata
• SQLAlchemy default convention ignored
• Fully dynamic templates (no literal prefix/suffix) skipped gracefully
• make fmt && make test green (78 passed, coverage 63.96%)

[codecov]

Codecov Report

❌ Patch coverage is 6.66667% with 42 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
alembic_gauntlet/utils/convention.py	6.66%	42 Missing ⚠️

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