Improve CountdownHelper: optional messages, finishMessage, and boss bar color

[Copilot]

CountdownHelper lacked a finish-state message, forced callers to always provide a message string, and hardcoded the boss bar color to blue.

Changes

start() signature

• message: String? = null — per-tick display is skipped when omitted
• finishMessage: String? = null — shown via displayLocation at countdown end (replaces the would-be 0s tick); omit to suppress
• bossBarColor: BossBar.Color = BossBar.Color.BLUE — controls boss bar color when displayLocation == BOSS_BAR
• sound and finishSound now default to null; no need to pass them explicitly for silence
• displayLocation moved before the optional parameters for a cleaner call-site read order

Internal

• Extracted private showMessage() helper — eliminates duplication between the per-tick and finish display paths

Example

CountdownHelper().start(
    plugin          = plugin,
    player          = player,
    seconds         = 10,
    displayLocation = DisplayLocation.BOSS_BAR,
    bossBarColor    = BossBar.Color.RED,
    message         = "<yellow>Starts in {seconds}",
    finishMessage   = "<green>Go!",
    finishSound     = Sound.sound(Key.key("minecraft:entity.player.levelup"), Sound.Source.MASTER, 1f, 1f),
    onFinish        = { p -> startGame(p) }
)