Autocomplete: set the default anthropic model only for certain cases

[valerybugakov]

• Fixes the issue when enterprise instances use the non-deployed Anthropic model as a default value. Now we set the default Anthropic model only for DotCom users and only in cases where the autocomplete provider is set to Anthropic in local settings. It keeps the original issue fixed and also addresses the enterprise problem.
• Slack investigation here.
• Follow-up issues:
  • Write unit/integration tests for ModelsService and autocomplete: https://linear.app/sourcegraph/issue/CODY-3030/write-test-for-modelsservice-usage-with-autocomplete-on-the-client
  • Update terminology by removing multiple entities named provider: https://linear.app/sourcegraph/issue/CODY-3031/update-terminology-by-removing-multiple-entities-named-provider-in
  • Update the logic to pass model IDs to the Anthropic provider for all the cases where sourcegraph is a provider: https://linear.app/sourcegraph/issue/CODY-3032/update-the-logic-to-pass-model-ids-to-anthropic-provider-for-all-the

Test plan

For PLG:

1. Connect to DotCom using Cody VS Code
2. Set autocomplete provider to Anthropic "cody.autocomplete.advanced.provider": "anthropic"
3. Generate a completion.

For enterprises:

1. Connect to https://sg02.sourcegraphcloud.com/
2. Generate a completion.