Fix HTTP test port race and self-request origin

[ChihweiLHBird]

Summary

Fixes flaky HTTP integration test startup caused by a test harness port race.

The harness previously selected a port by binding localhost:0, reading the assigned port, dropping that listener, and later launching spin up --listen 127.0.0.1:{port}. In parallel test runs, another process could claim that port before this Spin instance bound it. The old readiness check only proved that something accepted TCP connections on the port, so a test could accidentally send requests to the wrong Spin app and see a spurious 404.

Change

Production code

• Track the actual bound HTTP listener address and use it for self-request origin setup.

Test framework

• Start Spin with --listen 127.0.0.1:0, then parse the actual bound port from this Spin process's startup output.
• Report early Spin startup failures with stdout/stderr.

Why the server change is needed

After the test harness switched to 127.0.0.1:0, HttpServer::listen_addr could remain 127.0.0.1:0 even though the actual listener was bound to an assigned port. Self requests must use the real bound address, not the configured :0 address. The server now stores listener.local_addr() after binding and uses that address when configuring SelfRequestOrigin.

For in-process tests, InProcessSpin builds a server and drives handle() directly without calling serve(), so no socket address is recorded. That path falls back to the configured listen address.