chore(spanner): use nam6 instance config for test_create_instance_partition

[sakthivelmanii]

Cloud Spanner enforces DuplicatedBaseInstancePartitionConfigs ('Cannot create an instance partition that shares a base config with another instance partition.').

Since snippets.create_instance_partition creates an instance partition in nam3, update test_create_instance_partition in snippets_test.py to create the parent instance in nam6 rather than nam3. This satisfies the multi-region parent instance requirement while avoiding base config collision.

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request updates the test_create_instance_partition test in snippets_test.py by removing the multi_region_instance_config fixture parameter and hardcoding the instance configuration path to use the nam6 multi-region configuration. There are no review comments, and I have no additional feedback to provide.