feat: support to reflection v2 api

[xavidop]

Description

Support for Reflection V2 in Genkit Java

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

Documentation

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

Reviewer Notes

[gemini-code-assist]

Code Review

This pull request introduces ReflectionServerV2, which enables the Genkit runtime to act as a WebSocket client for dev tools integration using JSON-RPC 2.0. The Genkit class is updated to conditionally start this server based on the GENKIT_REFLECTION_V2_SERVER environment variable. Feedback identifies a critical bug in handleCancelAction where all active actions are interrupted instead of the specific one requested, and points out an unused stream variable in handleRunAction.

[pavelgj]

Id needs to be globally unique, especially when multiple Genkit instances are involved.
Generally the logic is that runtime consists of: $prefix$suffix
Prefix is chosen based in the order:

1. genkit instance name
2. pid

Suffix: If it's the first genkit instance (based on singleton global counter), then empty suffix, otherwise it's _$instanceCount