Test Connection sends live API request on every click

[AshayK003]

Problem

sidebar.py:124-138 sends a real POST to the LLM endpoint with the user's API key every click. For metered providers, this consumes quota.

Solution

Cache the test result and prevent repeated tests with the same config.

Acceptance Criteria

• Test Connection doesn't fire duplicate requests with same provider/model/key

Difficulty: Easy