test(ircserver): cover VERSION/ADMIN/TIME under the default empty SERVER_NAME

Both wire test environments hardcoded ServerName: "test.irc", so no test
exercised the shipped default and the empty server-name parameter went
unnoticed for five rework rounds. Parameterize the env's server name and
add a wire test that runs with it empty, asserting the numerics name
"neoirc" and contain no empty parameter. Verified to fail against the
pre-fix handlers.

internal/ircserver/server_test.go

@@ -38,6 +38,20 @@ type testEnv struct {
 func newTestEnv(t *testing.T) *testEnv {
 	t.Helper()
 
+	return newTestEnvWithServerName(t, "test.irc")
+}
+
+// newTestEnvWithServerName creates a test environment with
+// an explicit SERVER_NAME. Passing "" exercises the shipped
+// default from config.go, under which the server must fall
+// back to "neoirc" rather than emitting an empty
+// server-name parameter.
+func newTestEnvWithServerName(
+	t *testing.T,
+	serverName string,
+) *testEnv {
+	t.Helper()
+
 	dsn := fmt.Sprintf(
 		"file:%s?mode=memory&cache=shared&_journal_mode=WAL",
 		t.Name(),
@@ -67,7 +81,7 @@ func newTestEnv(t *testing.T) *testEnv {
 	brk := broker.New()
 
 	cfg := &config.Config{ //nolint:exhaustruct
-		ServerName: "test.irc",
+		ServerName: serverName,
 		MOTD:       "Welcome to test IRC",
 	}