feat: skip saving worlds on shutdown when autosave is disabled

[DartCZ]

Summary

Respect the ticks-per.autosave setting on server shutdown. When autosave is
disabled (ticks-per.autosave <= 0), Paper no longer writes world chunks to
disk during stopServer() it only closes the chunk system cleanly. When
autosave is enabled (> 0), behaviour is unchanged: worlds are saved on stop
exactly as before.

Motivation

Currently ticks-per.autosave <= 0 disables periodic autosaving, but the
server still performs a full world save on shutdown, so the setting does not
actually guarantee a non-persistent world.

This is most useful for minigame / lobby servers whose main world is meant
to be ephemeral: the map is reset from a template on every boot, so persisting
player-made changes on shutdown is undesirable (and a waste of disk I/O on large
voided worlds). With this change, setting ticks-per.autosave to 0/-1 makes
the world truly read-only across restarts, without resorting to plugins that
force-unload or wipe regions.

The boundary (> 0 saves, <= 0 skips) matches the existing convention already
used elsewhere in the server:

• MinecraftServer#tickServer - fullSave = this.autosavePeriod > 0 && ...
• CraftServer#checkSaveState - treats autosavePeriod <= 0 as disabled.

Scope / notes

• This only gates chunk saving. Global level metadata (level.dat,
  scoreboard, saved data via saveGlobalData) and player data are still saved
  on stop, as before. Gating those is intentionally out of scope. (Extra option for this in the future?)
• Also removes an adjacent dead while (false && ...) block left over from the
  chunk-system rewrite, since the surrounding code was already being touched.

[Warriorrrr]

Disabling autosaving should not disable world saving entirely

[DartCZ]

Disabling autosaving should not disable world saving entirely

Agree and to clarify, this PR doesn't disable world saving entirely.
Explicit saves are completely unaffected:

• /save-all calls saveEverything(..., force = true), which ignores the
  noSave flag, so it always writes , even with autosave disabled.
• World.save() / plugin-triggered saves go through the same force path.

[Toffikk]

these changes should be merged into source/respective patches

[DartCZ]

these changes should be merged into source/respective patches

Done ✅