Feature Request: JetStream Consumer Support via DSL #44

Feature Request: JetStream Consumer Support via DSL

Background

Leopard's DSL is elegant and safe for NATS request/reply — it handles threading through a Concurrent::FixedThreadPool, enforces Dry::Monads::Result return values, and removes entire categories of concurrency bugs. The endpoint block pattern is easy to reason about and hard to misuse.

However, Leopard currently has no support for NATS JetStream consumers. Users who need async, durable message processing (e.g., work queues, event-driven pipelines, at-least-once delivery) must drop down to raw nats-pure JetStream code. This means hand-rolling thread management, ack/nak handling, and consumer lifecycle — all the things Leopard protects against in the request/reply world.

Problem

When a team adopts Leopard for its safety guarantees and then needs JetStream, they hit a cliff. There is no way to express a JetStream consumer in the DSL, so they write a full handler class from scratch. This reintroduces the threading pitfalls Leopard was designed to eliminate, and fragments the codebase between "Leopard-style" and "raw NATS" services.

Proposed Solution

The framework would:

• Ack on Success, Nak on Failure (with configurable backoff / retry limit)
• Term on unhandled exception (to avoid poison-pill loops), with optional rescue callback
• Reuse the existing Concurrent::FixedThreadPool / instances: scaling model
• Pass the same MessageWrapper interface so middleware and existing handler patterns are unchanged

Why This Matters

• Users already inside the Leopard ecosystem shouldn't need to leave it to do async work
• The threading model that makes request/reply safe applies equally to JetStream — there is no reason to give that up
• Consistent use of Dry::Monads::Result across both sync and async handlers reduces surface area for bugs
• Middleware (auth, logging, metrics) would apply uniformly without extra wiring

Acceptance Criteria

• jetstream_endpoint DSL method (or equivalent) with stream:, subject:, and durable: options
• Automatic ack on Success, nak on Failure, term on unhandled exception
• Works with the existing instances: concurrency model
• Pull consumer support
• Example service in examples/