Update projects API to filter by projects with code embeddings

Context

As part of Semantic search: chat with your codebase (&16910), the Language Server will use the GET /projects API to fetch projects to be displayed on the /include <repository> menu

We need to support a new parameter on this API to allow filtering by projects with code embeddings.

References

• initial discussions for this requirement: &17298 (comment 2519369936)
• see Index state management section of the Code Embeddings blueprint
• issue for creating the Index state management tables: [Index state tracking] Create tables for tracki... (#536221 - closed)
• see conversation about state tracking: gitlab-com/content-sites/handbook!12808 (comment 2537746234)

Proposal

• Update the ProjectsFinder to accept a boolean with_code_embeddings_indexed parameter, and filter by p_ai_active_context_code_repositories.state=ready (see discussion)
• Update the GET /projects to accept a boolean with_code_embeddings_indexed and pass it on to the ProjectsFinder