CosmosDB Test Fixture should be dynamic and match bicep file

For the Cosmos test fixture where we populate the DB with dummy items, we should avoid having hardcoded names. Instead, we should retrieve the name of the DB and Container from a variable to ensure we're using the same one as the bicep files and post-scripts for the test resources.

See https://github.com/Azure/azure-mcp/pull/304#discussion_r2138974931