test(spanner): disable pooling in mock server tests

[chalmerlowe]

This PR disables connection pooling in the mock server tests for sqlalchemy-spanner.

We observed that many tests were failing with an off-by-one error in the expected number of requests (e.g., expecting 6 but getting 5). This suggests that CreateSessionRequest was not being sent for every test, likely because sessions were being reused from the pool across tests.

By setting poolclass=NullPool in create_engine, we force a new connection and session for every test, ensuring predictable request counts.

[gemini-code-assist]

Code Review

This pull request reorganizes imports and disables connection pooling in the create_engine method by using NullPool to ensure fresh sessions for each test. The review feedback recommends further organizing the import block to comply with PEP 8 alphabetical sorting and moving the NullPool import from the local method scope to the top level to improve code maintainability.

[gemini-code-assist]

The import block is disorganized and does not follow PEP 8 alphabetical sorting. Additionally, NullPool should be imported at the top level rather than locally within a method to improve readability and maintainability.

Suggested change

	import google.cloud.spanner_v1.types.result_set as result_set
	import google.cloud.spanner_v1.types.type as spanner_type
	import grpc
	from google.api_core.client_options import ClientOptions
	from google.auth.credentials import AnonymousCredentials
	from google.cloud.spanner_dbapi.parsed_statement import AutocommitDmlMode
	from google.cloud.spanner_v1 import (
	Client,
	ResultSet,
	TypeCode,
	)
	from google.cloud.spanner_v1.database import Database
	from google.cloud.spanner_v1.instance import Instance
	import grpc
	from sqlalchemy import Engine, create_engine
	from sqlalchemy.testing.plugin.plugin_base import fixtures
	from google.api_core.client_options import ClientOptions
	from google.auth.credentials import AnonymousCredentials
	from google.cloud.spanner_dbapi.parsed_statement import AutocommitDmlMode
	from google.cloud.spanner_v1 import (
	Client,
	ResultSet,
	TypeCode,
	)
	from google.cloud.spanner_v1.database import Database
	from google.cloud.spanner_v1.instance import Instance
	import google.cloud.spanner_v1.types.result_set as result_set
	import google.cloud.spanner_v1.types.type as spanner_type
	import grpc
	from sqlalchemy import Engine, NullPool, create_engine
	from sqlalchemy.testing.plugin.plugin_base import fixtures

[gemini-code-assist]

Remove the local import of NullPool as it has been moved to the top-level imports.

        return create_engine(