Fix Prometheus integration payload and validation endpoints

[Rohithmatham12]

Summary

• send Prometheus tenant/default fields with the same camelCase shape used by the API and update command
• keep password in the generated add payload and add regression assertions for the request body
• correct Prometheus validate endpoints to the singular configuration validate routes covered by tests

Fixes #136.

Testing

• CORTEX_BASE_URL=http://localhost CORTEX_API_KEY=test POETRY_CACHE_DIR=/private/tmp/cortexapps-cli-poetry-cache poetry run pytest tests/test_integrations_prometheus.py -k "not add_multiple" -q