feat(go): added support for durable streaming in flows

[apascal07]

Adds support for durable streaming, allowing clients to reconnect to in-progress or completed streams using a stream ID.

Usage:

sm := core.NewInMemoryStreamManager(core.WithTTL(10 * time.Minute))
handler := genkit.Handler(myStreamingFlow, genkit.WithStreamManager(sm))

Behavior:

• Streaming responses include X-Genkit-Stream-Id header
• Clients can subscribe by sending X-Genkit-Stream-Id in request header
• Subscribing replays buffered chunks then continues with live updates
• Subscribing to non-existent stream returns 204 No Content

Non in-memory implementations like Firestore to follow.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Summary of Changes

Hello @apascal07, 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 enhances the streaming capabilities of Go flows by introducing durable streaming. This feature allows clients to seamlessly reconnect to ongoing or completed streaming operations, improving the robustness and user experience of applications that rely on real-time data. It provides the necessary interfaces, an in-memory implementation, and integrates this functionality directly into the existing handler mechanism, making it easy to adopt.

Highlights

• Durable Streaming Interfaces: Introduced StreamManager and ActionStreamInput interfaces to define the contract for managing and interacting with durable streams, allowing for flexible storage backends.
• In-Memory Stream Manager: Provided an InMemoryStreamManager implementation, complete with options like WithTTL for automatic cleanup of completed streams, suitable for testing or single-instance deployments.
• Handler Integration: Added a WithStreamManager option to genkit.Handler to enable durable streaming for flows, automatically managing stream IDs and client reconnections.
• Client Reconnection: Clients can now reconnect to in-progress or completed streams by sending an X-Genkit-Stream-Id header, which replays buffered chunks and continues with live updates or the final result.
• New Sample Application: A new sample (durable-streaming/main.go) demonstrates how to set up and use durable streaming with a countdown flow, including curl examples for testing.

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[gemini-code-assist]

Code Review

This pull request introduces a significant and well-designed feature for durable streaming in Go flows. The new interfaces and the in-memory implementation are clear and robust. The integration into the HTTP server is also well-handled. I've identified a critical issue regarding error handling in durable writes, a high-severity issue with the memory cleanup strategy that could lead to a memory leak in long-running services, and a minor issue regarding a magic number. Overall, this is a great addition, and with these fixes, it will be even more solid.

[pavelgj]

some unit tests for the in memory stream manager?