fix(engine, filesystem-http): reset config watch keys after every event

[akrambek]

Fixes #2624

Problem

• EngineConfigWatchTask.call() loops on watcher.take() and calls onPathChanged(...), but never calls pollEvents() or reset() on the returned key. A signalled WatchKey is not re-queued until it is reset. When an event leaves the config text unchanged, EngineManager.onPathChanged returns early, nothing re-arms the key, and every later config change is ignored until restart.
• HttpWatchKey (filesystem-http) does not implement reset(): it throws UnsupportedOperationException. So once the watch loop resets keys, http-sourced configs stop reloading after the first change.
• HttpWatchKey.cancel() throws NullPointerException if the key has not sent its first watch request yet. A reload that unregisters such a key fails, so the new config is rejected.

Change

• engine: after every take(), drain key.pollEvents(), call onPathChanged(...), then call key.reset(). This happens whether or not the config changed. If reset() reports the config path key is no longer valid, register the config path again.
• filesystem-http: HttpWatchKey now follows the WatchKey contract.
  • A key is queued once when it is first signalled.
  • reset() re-queues it only if events are still pending, and returns false once the key is cancelled.
  • cancel() works before the first watch request, and a cancelled key sends no further watch requests.

Consumers of HttpWatchService must now call reset() after handling a key, as the WatchKey contract requires. Before this change, a key was queued again on every event.

Tests

• EngineConfigWatchTaskTest.shouldApplyChangeAfterUnchangedRewrite: rewrite the local config with the same text, then change it; the change is applied.
• EngineConfigWatchTaskTest.shouldApplyHttpChangesAfterReload: an http-sourced config (local long-poll server with etags) changes twice, and both changes are applied.
• HttpFileSystemTest.shouldCancelWatchKeyBeforeWatching: cancelling right after register does not throw; the key is invalid and reset() returns false.
• HttpFileSystemIT.shouldWatch: now calls reset() between the two take() calls and checks that it returns true.

Each new test failed before its fix and passes after it.

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