test(spanner): replace DB-API DDL system tests with mock server tests

[olavloite]

Replaces live-instance DDL system tests in test_dbapi.py with fast, deterministic in-memory mock server tests in test_dbapi_ddl.py.

Previously, test_dbapi.py dynamically created and dropped ephemeral tables (e.g. Singers, DdlExecute, DdlExecuteMany, JsonDetails) against live Cloud Spanner instances to verify client-side DDL buffering and flushing. Each table creation/drop triggered a long-running operation (LRO), adding ~1.5 to 2.0 minutes of latency and causing test flakiness.

Changes:

• Add test_dbapi_ddl.py in tests/mockserver_tests/ with 9 mock server tests covering immediate autocommit DDL execution, non-autocommit DDL buffering, multi-statement DDL batches, DDL flushing before subsequent queries and DML, and client-side rejection of DDL in executemany().
• Remove 7 redundant DDL tests from tests/system/test_dbapi.py.
• Update test_autocommit_with_json_data in test_dbapi.py to reuse the existing all_types table with row cleanup instead of creating and dropping a dedicated table.
• Eliminate all dynamic table churn in DB-API system tests, saving ~1.5 to 2.0 minutes in standard CI while executing the replacement mock tests in < 1 second.

[gemini-code-assist]

Code Review

This pull request migrates DB-API DDL tests from system tests to mock server tests, introducing a comprehensive suite in test_dbapi_ddl.py and removing redundant tests from test_dbapi.py. Additionally, test_autocommit_with_json_data is refactored to use an existing table. Feedback on the changes highlights a potential state leak in test_autocommit_with_json_data where self._conn.autocommit is modified but not restored, which could affect subsequent tests sharing the same connection.