Fix Echo test provider to respect default model preference setting

[cameronmpalmer]

Summary

• Fix Echo language model provider ignoring the user's models.preference.byProvider setting
• The provider was hardcoding isDefault: true on the first model instead of using markDefaultModel() like other providers

QA Notes

To test:

1. Add the Echo provider via "Positron Assistant: Configure Models"
2. Set a preference in settings:

   "positron.assistant.models.preference.byProvider": {
       "echo": "v2"
   }

3. Reload the window
4. Open the Assistant panel and check the model picker - "Echo Language Model v2" should show "(default)" indicator

[sharon-wang]

Thanks for fixing this up!

This particular file is being reorganized into multiple files in #11089. It's a big refactor PR that should be going in within the next day or so. Would you be up for waiting for that PR to be merged, and porting this change over to the new file (should be in extensions/positron-assistant/src/providers/test/echoProvider.ts)?

[cameronmpalmer]

Thanks for fixing this up!

This particular file is being reorganized into multiple files in #11089. It's a big refactor PR that should be going in within the next day or so. Would you be up for waiting for that PR to be merged, and porting this change over to the new file (should be in extensions/positron-assistant/src/providers/test/echoProvider.ts)?

Can do!

[cameronmpalmer]

@sharon-wang Ported to non-fork PR with refactored changes in #11362