fix: refresh codebase index on config change

[shssoichiro]

Description

Updates the CodebaseIndexer to refresh the index when the assistant config is changed. The DocsService already does this, so this change copies the approach that the DocsService is using.

The biggest current issue is that the codebase indexing will begin before Continue has loaded remote assistant configs, which means it will use whatever model is available locally (usually Transformers.js in VSCode), or worse, silently fail. By refreshing after the config loads, we will ensure we are indexing the codebase using the user's configured embed model.

Checklist

• [x] I've read the contributing guide
• [x] The relevant docs, if any, have been updated or created
• [x] The relevant tests, if any, have been updated or created

Screenshots

N/A This is a backend change.

Tests

Added appropriate tests to CodebaseIndexer.test.ts