feat: add nats_jetstream_consumer to provide the feature requested in issue #44

[bougyman]

This add a NatsJetstreamConsumer class which provides the same
concurrency primitives and connection handling that the
NatsApiServer does for NATS service api processes. Example
included for usage.

closes #44

[Copilot]

Pull request overview

Adds first-class NATS JetStream pull-consumer support to Leopard’s DSL/runtime, reusing the existing middleware + Dry::Monads::Result handler contract and worker lifecycle model (issue #44).

Changes:

• Introduces jetstream_endpoint DSL plus NatsJetstreamConsumer to run pull subscriptions per worker instance.
• Extracts common message execution/middleware/result-routing into MessageProcessor, with separate callback sets for request/reply vs JetStream.
• Adds JetStream integration tests + example, and updates CI tooling to run tests against a managed local JetStream broker.

Reviewed changes

Copilot reviewed 15 out of 17 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
lib/leopard/nats_api_server.rb	Adds jetstream_endpoint DSL, wires JetStream consumer startup, and routes all message handling through MessageProcessor.
lib/leopard/message_processor.rb	New shared middleware/handler/result routing component used by both request/reply and JetStream flows.
lib/leopard/nats_jetstream_consumer.rb	New pull-consumer loop (threads, fetch, dispatch) for JetStream endpoints.
lib/leopard/nats_jetstream_callbacks.rb	New JetStream ack/nak/term callbacks based on handler outcomes.
lib/leopard/nats_jetstream_endpoint.rb	New struct describing JetStream endpoint configuration/options.
lib/leopard/nats_request_reply_callbacks.rb	New request/reply callbacks (respond, respond_with_error) extracted from prior inline logic.
lib/leopard.rb	Exposes new components from the top-level require.
test/lib/nats_api_server.rb	Updates tests to exercise jetstream_endpoint registration and MessageProcessor routing behavior.
test/lib/nats_request_reply_callbacks_test.rb	New unit tests for request/reply callback behavior.
test/integration/nats_jetstream_integration_test.rb	New integration coverage for ack-on-success, nak/redelivery-on-failure, term-on-exception, and middleware execution.
examples/jetstream_endpoint.rb	Usage example for defining and running a JetStream consumer endpoint.
Rakefile	Adds rake ci orchestration to start/stop a local NATS JetStream container and run tests.
ci/nats/start.sh	Adds configurable container name/detach mode and builds docker/podman args more flexibly.
.github/workflows/main.yaml	Switches CI to rake ci and uses local container startup instead of GH Actions “services”.
Readme.adoc	Documents JetStream pull consumer DSL and updates dev instructions to use rake ci.
Gemfile.lock	Bumps gem version to 0.2.5 in the lockfile.
.gitignore	Ignores .rubocop_cache.

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