Function DiscoverComposerTriggerProviders

Resolves every @RegisterClass(ComposerTriggerProvider, ...) registration in the MJ ClassFactory into a provider instance list.

• Instantiates each winning registration once (module-level cache keyed by the registered subclass) — repeated calls are cheap.
• When multiple registrations share a Key, the ClassFactory's highest-priority registration wins (standard override-by-import-order semantics).
• excludedKeys filters providers out by their stable Key (case-insensitive).
• Returns [] gracefully when nothing is registered — the editor then behaves as a plain text editor.

Results are sorted by provider Priority (desc), then Key (asc) for stability.