feat(genkit-tools): add /listValues endpoint to expose defaultModel

[ssbushi]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually)

[gemini-code-assist]

Summary of Changes

Hello @ssbushi, 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 introduces a new API endpoint and associated functionality within the genkit-tools to allow for the retrieval of various runtime-specific values. This enhancement provides a standardized way to query and expose internal Genkit data, such as defaultModel (implied by the title, though the code uses a generic type parameter), making it more accessible for tooling and introspection.

Highlights

• New /listValues Endpoint: A new /listValues endpoint has been introduced in genkit-tools to retrieve various runtime-specific values, enhancing the ability to inspect Genkit application states.
• RuntimeManager Enhancement: The RuntimeManager now includes a listValues method, responsible for orchestrating the fetching of values from the reflection server, handling runtime selection and potential errors.
• API Schema Definition: A new ListValuesRequestSchema and corresponding type have been defined, standardizing the input for the new endpoint, which requires a type parameter and optionally accepts a runtimeId.
• ReflectionServer Integration: The ReflectionServer now exposes a /api/values GET endpoint, allowing clients to query for specific types of values, such as defaultModel, by providing a type query parameter.

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[gemini-code-assist]

Code Review

This pull request adds a new /listValues endpoint to expose values from the registry. The changes are mostly correct, but I've identified a bug in the client-side request construction within RuntimeManager and a minor issue with error handling on the server-side. My review includes specific suggestions to address these points for improved correctness and robustness.

[ssbushi]

Restricted to defaultModel only.

[ssbushi]

Realized that having the conversion done at registry time is more convenient, since all types needed are available here. WDYT?

[pavelgj]

backwards compatibility consideration -- Go and python won't have this endpoint implemented, so will be getting 404. Should 404 be handled here and an empty record returned, or bubble up the error all the way to the Dev UI and let it deal with backwards compatibility?

I'm leaning towards handle 404 here...