feat(aigw): consolidate admin server into a single port (#1236)

**Description**

This consolidates the extproc admin server from two separate ports
(metricsPort and healthPort) into a single adminPort (default 1064)
serving both /metrics and /health endpoints. The `aigw run` and the new
`aigw healthcheck` commands now take --admin-port flags, propagating
this to extproc.

Previously, extproc exposed metrics on port 1064 and health checks on
port 1065. Now both are served from a single HTTP server on port 1064,
simplifying configuration and Docker HEALTHCHECK setup.

The Docker HEALTHCHECK is now functional, using the new healthcheck
subcommand which polls the admin server's /health endpoint. This /health
endpoint proxies to the extproc gRPC health check, returning HTTP 200
when SERVING. A new subcommand (`aigw healthcheck`) was needed as the
image is distroless (has no curl etc).

When aigw run starts Envoy, it polls for readiness using either the
Envoy admin address (when configured in EnvoyProxy bootstrap) or by
checking if the Gateway listener port accepts TCP connections. This
ensures startup completes reliably with minimal Gateway configurations.

Gateway validation now requires at least one listener to be configured.

---------

Signed-off-by: Adrian Cole <[email protected]>

Author: codefromthecrypt

tests/internal/e2elib/e2elib.go

@@ -906,3 +906,29 @@ func waitUntilKubectl(t *testing.T, timeout time.Duration, pollInterval time.Dur
 	}
 	require.Fail(t, "timed out waiting", "last error: %v", lastErr)
 }
+
+// waitUntilKubectl polls by running a kubectl command with the given args
+// until the verifyOut predicate returns nil or the timeout is reached.
+//
+// Unlike require.Eventually, this retains the output of the last mismatch.
+func waitUntilKubectl(t *testing.T, timeout time.Duration, pollInterval time.Duration, verifyOut func(output string) error, args ...string) {
+	var lastErr error
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		cmd := Kubectl(t.Context(), args...)
+		cmd.Stdout = nil // To ensure that we can capture the output by Output().
+		out, err := cmd.Output()
+		if err != nil {
+			lastErr = err
+			time.Sleep(pollInterval)
+			continue
+		}
+		err = verifyOut(string(out))
+		if err == nil {
+			return
+		}
+		lastErr = err
+		time.Sleep(pollInterval)
+	}
+	require.Fail(t, "timed out waiting", "last error: %v", lastErr)
+}