feat: add middelwares to the ui

[xavidop]

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvements
• CI/CD improvements

Changes Made

Add middleware support to the Dev UI

Checklist

• My code follows the project's code style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Screenshots (if applicable)

Documentation

• README updated
• Plugin documentation updated
• API documentation updated
• Sample code updated

Reviewer Notes

[gemini-code-assist]

Code Review

This pull request introduces support for registering and resolving generation middleware from the Genkit Dev UI. It adds a new use field to GenerateActionOptions to receive selected middleware names, resolves these names to fresh middleware instances in GenerateAction, and executes them through the wrapGenerate, wrapModel, and wrapTool chains. It also updates the registry to support typed value buckets (e.g., "middleware"), exposes these values via the reflection API endpoints (/api/values), and provides builder methods to pre-register middleware. The reviewer feedback highlights three key issues: a concurrency bug in Genkit.registerMiddlewareForDevUi due to a non-atomic check-then-act pattern, a thread-safety issue in DefaultRegistry.registerValue that can be resolved using putIfAbsent, and a potential deserialization issue in GenerateAction when converting tool inputs, suggesting the use of JsonUtils.convert(...) instead of the local objectMapper.