Update agent disable/enable tests, following service fix

[dargilco]

This pull request updates the agent enable/disable logic and refines the associated tests to ensure correct error handling and endpoint coverage. It also updates .env.template with missing environment variables.

I verified that the 2 bugs I reported were fixed:

1. Service now returns 204 instead of 200 on success of agent enable/disable, as per TypeSpec
2. Agent enable/disable works on Project Endpoint as well as Agent Endpoint. Previously it only worked on Agent Endpoint.

Agent enable/disable logic and error handling:

• Changed the allowed HTTP status codes for agent enable and disable operations to only accept 204 (No Content), ensuring stricter compliance with expected API responses. [1] [2] [3] [4]
• Updated both sync and async tests in test_agents_crud.py and test_agents_crud_async.py to:
  • Use the input parameter instead of creating conversations for agent response tests.
  • Explicitly check for PermissionDeniedError when calling a disabled agent, ensuring the agent properly rejects requests when disabled. [1] [2] [3] [4] [5] [6]
  • Clean up any existing agents before running tests to avoid conflicts. [1] [2]

Test coverage improvements:

• The sync test now uses the agent endpoint and the async test uses the project endpoint for responses, ensuring both code paths are exercised. [1] [2]

Configuration and asset updates:

• Added FABRIC_IQ_PROJECT_CONNECTION_ID and FABRIC_IQ_USER_INPUT to .env.template for new configuration options.
• Updated the asset tag in assets.json to a new version.