Preserve assistant messages with tool calls

[guerler]

Newer models can return assistant messages with both content and tool_calls. The plugin chat adapter now forwards both fields so tool-based workflows keep working.

The PR also improves error handling for JupyterLite by showing the actual provider error message instead of a generic one.

Before:

Now:

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[guerler]

Failing tests are unrelated.