refactor: replace all bare command string literals with named constants

Extract cmdLusers, cmdMode, cmdMotd, cmdNames, cmdNotice, cmdPing, cmdPong
constants in internal/handlers/api.go. Add corresponding constants in
cmd/neoirc-cli/main.go and cmd/neoirc-cli/api/client.go. Replace every bare
IRC command string literal in switch cases and command dispatch code with the
named constant. Zero bare command strings remain in any dispatch path.

internal/handlers/api.go

@@ -29,8 +29,15 @@ const (
 	maxHistLimit       = 500
 	cmdJoin            = "JOIN"
 	cmdList            = "LIST"
+	cmdLusers          = "LUSERS"
+	cmdMode            = "MODE"
+	cmdMotd            = "MOTD"
+	cmdNames           = "NAMES"
 	cmdNick            = "NICK"
+	cmdNotice          = "NOTICE"
 	cmdPart            = "PART"
+	cmdPing            = "PING"
+	cmdPong            = "PONG"
 	cmdPrivmsg         = "PRIVMSG"
 	cmdQuit            = "QUIT"
 	cmdTopic           = "TOPIC"
@@ -758,7 +765,7 @@ func (hdlr *Handlers) dispatchCommand(
 	bodyLines func() []string,
 ) {
 	switch command {
-	case cmdPrivmsg, "NOTICE":
+	case cmdPrivmsg, cmdNotice:
 		hdlr.handlePrivmsg(
 			writer, request,
 			sessionID, clientID, nick,
@@ -789,7 +796,7 @@ func (hdlr *Handlers) dispatchCommand(
 		hdlr.handleQuit(
 			writer, request, sessionID, nick, body,
 		)
-	case "MOTD", cmdList, cmdWho, cmdWhois, "PING":
+	case cmdMotd, cmdList, cmdWho, cmdWhois, cmdPing:
 		hdlr.dispatchInfoCommand(
 			writer, request,
 			sessionID, clientID, nick,
@@ -812,13 +819,13 @@ func (hdlr *Handlers) dispatchQueryCommand(
 	bodyLines func() []string,
 ) {
 	switch command {
-	case "MODE":
+	case cmdMode:
 		hdlr.handleMode(
 			writer, request,
 			sessionID, clientID, nick,
 			target, bodyLines,
 		)
-	case "NAMES":
+	case cmdNames:
 		hdlr.handleNames(
 			writer, request,
 			sessionID, clientID, nick, target,
@@ -839,7 +846,7 @@ func (hdlr *Handlers) dispatchQueryCommand(
 			writer, request,
 			sessionID, clientID, nick, target,
 		)
-	case "LUSERS":
+	case cmdLusers:
 		hdlr.handleLusers(
 			writer, request,
 			sessionID, clientID, nick,
@@ -1571,7 +1578,7 @@ func (hdlr *Handlers) dispatchInfoCommand(
 	okResp := map[string]string{"status": "ok"}
 
 	switch command {
-	case "MOTD":
+	case cmdMotd:
 		hdlr.deliverMOTD(
 			request, clientID, sessionID, nick,
 		)
@@ -1589,10 +1596,10 @@ func (hdlr *Handlers) dispatchInfoCommand(
 			request, clientID, sessionID, nick,
 			target, bodyLines,
 		)
-	case "PING":
+	case cmdPing:
 		hdlr.respondJSON(writer, request,
 			map[string]string{
-				"command": "PONG",
+				"command": cmdPong,
 				"from":    hdlr.serverName(),
 			},
 			http.StatusOK)
@@ -1892,7 +1899,7 @@ func (hdlr *Handlers) handleMode(
 	if target == "" {
 		hdlr.respondIRCError(
 			writer, request, clientID, sessionID,
-			"461", nick, []string{"MODE"},
+			"461", nick, []string{cmdMode},
 			"Not enough parameters",
 		)
 
@@ -1981,7 +1988,7 @@ func (hdlr *Handlers) handleNames(
 	if target == "" {
 		hdlr.respondIRCError(
 			writer, request, clientID, sessionID,
-			"461", nick, []string{"NAMES"},
+			"461", nick, []string{cmdNames},
 			"Not enough parameters",
 		)