test: add always-on JetStream integration gate

[ZhiXiao-Lin]

Summary

• add ordinary push and pull-request CI for formatting, strict Clippy, the feature matrix, rustdoc, and package verification
• run the JetStream suite against the official NATS 2.11.8 image pinned by multi-architecture digest
• make the integration entry point fail closed when JetStream is required, tighten delivery assertions, and prove redelivery after acknowledgement loss
• fix defects exposed by the real gate: wildcard-safe durable consumer names and newest-first NATS history honoring limit
• repair small pre-existing lint, rustdoc, and hanging-test blockers required for a green quality gate

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets --all-features (196 unit, 29 memory integration)
• cargo test --all-targets --no-default-features (131 unit)
• RUSTDOCFLAGS="-D warnings" cargo doc --all-features --no-deps
• RUSTDOCFLAGS="-D warnings" cargo test --doc --all-features (2 passed)
• cargo package --allow-dirty
• NATS Server 2.11.8 with A3S_EVENT_REQUIRE_NATS=1 cargo test --test nats_integration -- --nocapture --test-threads=1 (11 passed)

Issue scope

This is the first always-on JetStream evidence slice for #1. It intentionally does not close the issue: server/client restart, reconnect bounds, credential rotation, consumer-drift detection, poison-message/DLQ behavior, end-to-end metadata/schema behavior, and the full observability acceptance criteria remain for follow-up work.

Pinned image: nats:2.11.8-alpine@sha256:71092f77d707a4a81b12aca5096d6b2d2e07ad16aa57c84066940a17af74f61a.

[ZhiXiao-Lin]

Review hardening added in the latest commit: JetStream history now scans the initial pending set in fixed 256-message pulls, keeps only the requested newest events, and applies one total request-timeout budget instead of sizing the client fetch buffer to the entire stream. The real JetStream ordering test now publishes 260 records so it crosses the batch boundary. Local all-feature tests (196 unit + 29 memory integration), strict Clippy, rustdoc, formatting, and diff checks pass; the hosted pinned-NATS job is the real-server evidence.