Fix E2E test gaps and add comprehensive E2E + parity test suites

- Fix pull consumer fetch: send original stream subject in HMSG (not inbox)
  so NATS client distinguishes data messages from control messages
- Fix MaxAge expiry: add background timer in StreamManager for periodic pruning
- Fix JetStream wire format: Go-compatible anonymous objects with string enums,
  proper offset-based pagination for stream/consumer list APIs
- Add 42 E2E black-box tests (core messaging, auth, TLS, accounts, JetStream)
- Add ~1000 parity tests across all subsystems (gaps closure)
- Update gap inventory docs to reflect implementation status

docs/plans/2026-03-12-e2e-tests-design.md

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+# NATS.E2E.Tests — Design
+
+**Date:** 2026-03-12
+**Status:** Approved
+
+## Overview
+
+A true black-box E2E test project that launches `NATS.Server.Host` as a child process and connects using only `NATS.Client.Core` from NuGet. No project references to any server code.
+
+## Components
+
+### 1. `NatsServerProcess` — Server Lifecycle Helper
+
+- Builds the host binary once (or locates pre-built output)
+- Launches `dotnet exec <path>/NATS.Server.Host.dll -p <port>` as a child process
+- Allocates a free port per instance
+- Waits for the server to be ready by polling TCP connect (with timeout)
+- On dispose: sends SIGINT / kills process, waits for exit
+- Captures stdout/stderr for diagnostics on failure
+
+### 2. `NatsServerFixture` — xUnit Collection Fixture
+
+- Implements `IAsyncLifetime`
+- Creates one `NatsServerProcess` per test class (or shared via collection)
+- Exposes `Port` and a `CreateClient()` helper that returns a connected `NatsConnection`
+
+### 3. Initial Tests
+
+- `ConnectAndPingTest` — Client connects, PING/PONG succeeds
+- `PubSubTest` — Two clients, publish on a subject, subscriber receives the message
+- `RequestReplyTest` — One client subscribes and replies, another sends a request and gets the response
+
+## Project Structure
+
+```
+tests/NATS.E2E.Tests/
+  NATS.E2E.Tests.csproj          # xUnit + NATS.Client.Core only, no server refs
+  Infrastructure/
+    NatsServerProcess.cs          # Process lifecycle management
+    NatsServerFixture.cs          # xUnit fixture
+  BasicTests.cs                   # Connect, PubSub, Request/Reply
+```
+
+## Key Decisions
+
+- **Binary discovery**: The fixture runs `dotnet build` on the host project and locates the output DLL via the known relative path from the solution root.
+- **Ready detection**: TCP connect poll to the allocated port with 10s timeout, 100ms interval.
+- **Process cleanup**: `Process.Kill()` with a graceful SIGINT attempt first.
+- **No shared state**: Each test class gets its own server process and port — full isolation.
+- **Timeout**: All async test operations use 10s CancellationTokenSource to avoid hangs.
+
+## Out of Scope (YAGNI)
+
+- No TLS/auth E2E tests in the initial project
+- No cluster/route/gateway/leaf-node multi-server tests
+- No JetStream E2E
+- No custom config file testing