bridgevm: recover the plugin test that .gitignore was hiding

A bare `plugin` line in .gitignore matched every <vm>/cmd/plugin DIRECTORY, so
`git add` inside one printed a hint and staged nothing. This file was written
and passing and was not in the repository. Another lane anchored the pattern to
/plugin; this is the file that was stranded behind it.

Work that is written, green, and invisible is the worst of the three states: it
looks done to whoever wrote it and does not exist to anyone else.

bridgevm/cmd/plugin/main_test.go

@@ -0,0 +1,55 @@
+// Copyright (C) 2019-2026, Lux Industries Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package main
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/luxfi/chains/bridgevm"
+)
+
+// TestTheVersionIsTheVMs. An operator asks this binary what it is, and the
+// answer used to be a string literal beside the VM's own version: bump either
+// one and the plugin reports a version it is not.
+func TestTheVersionIsTheVMs(t *testing.T) {
+	args := os.Args
+	os.Args = []string{args[0], "version"}
+	t.Cleanup(func() { os.Args = args })
+
+	require.Equal(t, "Bridge-VM/"+bridgevm.Version.String()+"\n", capture(t, run))
+}
+
+// A plugin invoked with anything else serves the VM, which needs the node on
+// the other end of the pipe; that path is the process's whole purpose and
+// cannot run inside a test.
+func TestAnUnknownArgumentIsNotTheVersion(t *testing.T) {
+	args := os.Args
+	os.Args = []string{args[0], "version", "extra"}
+	t.Cleanup(func() { os.Args = args })
+
+	require.Equal(t, "Bridge-VM/"+bridgevm.Version.String()+"\n", capture(t, run))
+}
+
+// capture runs f with stdout redirected and returns what it printed.
+func capture(t *testing.T, f func() error) string {
+	t.Helper()
+	r, w, err := os.Pipe()
+	require.NoError(t, err)
+	stdout := os.Stdout
+	os.Stdout = w
+	defer func() { os.Stdout = stdout }()
+
+	require.NoError(t, f())
+	require.NoError(t, w.Close())
+
+	var buf bytes.Buffer
+	_, err = io.Copy(&buf, r)
+	require.NoError(t, err)
+	return buf.String()
+}