Assistant: Remove testModels setting

[cameronmpalmer]

Fixes #11003

Summary

Removes the hidden positron.assistant.testModels setting which was causing confusion outside of testing. The setting was originally intended as a test shortcut but is no longer needed.

Changes

• Removed references to positron.assistant.testModels from production code:
  • src/vs/workbench/contrib/chat/browser/chatWidget.ts
  • src/vs/workbench/contrib/positronAssistant/browser/positronAssistantService.ts
• Updated e2e test fixtures to use positron.assistant.enabledProviders instead:
  • Added "echo" provider to test/e2e/fixtures/settings.json to enable test providers

Testing

Ran e2e tests for positron-assistant:

• 15 tests passed on e2e-electron (primary test platform)
• No new test failures introduced
• Pre-existing API key authentication test failures are unrelated to these changes

QA Notes

The echo provider has been added to enabledProviders in test fixtures as requested, enabling the echo models for automated testing.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[cameronmpalmer]

I have read the CLA Document and I hereby sign the CLA