feat: Add off mode to UI5_WATCH_MODE - #1565
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In CI and other environments where sources do not change while the server
runs, file watching is pure overhead. The polling backend is the worst case:
it walks the source tree every 250 ms, and it becomes the default inside
containers, where CI commonly runs.
UI5_WATCH_MODE=off makes the fileWatcher facade's subscribe() return an inert
subscription: the callback is never invoked and unsubscribe() is a no-op, so no
backend is loaded and no filesystem is polled. All three watcher consumers
(WatchHandler, ProjectDefinitionWatcher, projectGraphSettleWatcher) go through
this facade, so none of them arms an OS handle or a poll loop. Source changes
no longer trigger rebuilds or live reload for the server's lifetime.
The backend decision memoizes a mode string ("native" | "polling" | "off")
instead of a boolean; shouldUsePolling() and the new isWatchingDisabled()
derive from it.
…atching Add a Troubleshooting section pointing CI users at UI5_WATCH_MODE=off, next to the existing polling/native guidance. Names the polling watcher's tree walk and its container default as the overhead this avoids.
Keep the fileWatcher facade description in the skill reference in sync: note the "off" mode, the inert subscription it returns, and that the memoized decision is now a mode string behind shouldUsePolling()/isWatchingDisabled().
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This flag allows to disable file watching, e.g. in CI and other environments where sources do not change and the overhead (especially in polling mode) is unwanted.
JIRA: CPOUI5FOUNDATION-1355