[Do not merge] Fix FileSystemProvider reload race condition, redundant reloads, and missing warning

[strawgate]

The reload path in FileSystemProvider had two correctness issues and a usability gap. The component dict was cleared and rebuilt in place, so a concurrent reader could see an empty dict mid-reload. Every concurrent request also triggered its own full directory scan because the lock serialized but didn’t deduplicate. And a typo in the root path silently produced zero components.

Now _load_components builds into a fresh dict and swaps the reference atomically, a generation counter lets waiters skip redundant reloads, and a non-existent root logs a warning.

# Atomic swap — readers never see an empty dict
new_components = {}
self._components = new_components
# ... register into new_components ...

# Generation counter — deduplicate concurrent reloads
generation_before = self._reload_generation
async with self._reload_lock:
    if self._reload_generation == generation_before:
        self._load_components()
        self._reload_generation += 1

Closes #3625 (bugs 4, 5, and missing warning)

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