feat(ui): add missing embedding model error page

[kapetr]

Summary

Check the selected models against the actual agent demands, and provide an error message for a missing embedding model configuration, analogous to the existing error for a missing main model.

Linked Issues

Closes #1673

Documentation

• No Docs Needed: a fix

[gemini-code-assist]

Summary of Changes

Hello @kapetr, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the user experience during local development by introducing a dedicated error page for missing embedding model configurations, aligning with the existing error handling for large language models. It refactors the A2A client setup for better organization and introduces a flexible error display component, ensuring users receive clear guidance when essential models are not configured.

Highlights

• Embedding Model Error Page: Introduced a new error page specifically for cases where an embedding model is required but not configured, mirroring the existing LLM model error handling.
• Refactored A2A Client Initialization: The logic for initializing the A2A client and its fetchImpl has been extracted into a new file, agent-card.ts, promoting better modularity and reusability.
• Dynamic Model Selection Error Page: The NoModelSelectedErrorPage component was enhanced to dynamically display messages and setup commands for both LLM and embedding models, based on a new ModelType prop.
• Centralized Model Check Logic: A new utility function, ensureModelSelected, was created to centralize the validation of required LLM and embedding models, returning an appropriate error UI if any are missing.

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[gemini-code-assist]

Code Review

This pull request introduces a new feature to check for missing LLM or embedding model configurations and display a user-friendly error page. It also includes a good refactoring of the A2A client creation and error handling logic into a dedicated agent-card.ts file, improving modularity and reusability. The dynamic error messages for model selection are a nice touch for user experience.

[PetrBulanek]

It's not necessary to change it, but I personally would find it a little clearer if it returned the component, like this:

const { ErrorComponent } = await ensureModelSelected(providerId);

if (ErrorComponent) {
    return <ErrorComponent />;
}

At least at first glance, it indicates that something is being rendered. What do you think?

[kapetr]

I like that, I was also struggling with this not being obvious.