fix: make get_current_schema async to fix schema.table() alter migrations

[tmgbedu]

Summary

• get_current_schema() in PostgresPlatform, MySQLPlatform, and SQLitePlatform was calling connection.query(sql, ()), but Connection.query() takes no arguments — it's a factory that returns a QueryBuilder
• Any migration using async with await self.schema.table(...) to alter an existing table would crash with TypeError: Connection.query() takes 1 positional argument but 3 were given
• Fixed by making get_current_schema async and using await connection.select(sql, ()) instead
• Moved the schema fetch into Blueprint.__aexit__ so it runs asynchronously before to_sql() compiles the ALTER statements
• Updated test mocks from conn.query = MagicMock(return_value=[]) to conn.select = AsyncMock(return_value=[])
• Added regression test test_add_nullable_columns_without_from_table that reproduces the exact migration pattern that was failing

Reproduction

class AddCompanyProfileToClientsTable(Migration):
    async def up(self):
        async with await self.schema.table("clients") as table:
            table.text("website").nullable()
            table.text("linkedin_url").nullable()

Running this migration would raise:

TypeError: Connection.query() takes 1 positional argument but 3 were given

Test plan

• uv run pytest fastapi_startkit/tests/masoniteorm/sqlite/schema/test_sqlite_schema_builder_alter.py -v — all 14 tests pass including new regression test

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