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

Start with the Cosmos test fixture and review the linked pull request discussion for the existing hardcoded names. Trace the variables used by the bicep files and post-scripts, then update the fixture to use the same database and container names. Done means the dummy items target the resources defined for the test.

Written by the indexing model from the issue text.