fix(memory): make registerMemoryCorpusSupplement idempotent to prevent supplement loss during lazy re-register

[mgustimz]

Summary

When a plugin registers a MemoryCorpusSupplement from inside api.registerService({ start }), the supplement is correctly added to the global corpus supplements array.

However, when lazy tool-load triggers a subsequent non-activating re-register of the same plugin, the loader:

1. Snapshots the supplements BEFORE register
2. Runs register() which does NOT re-register the supplement (because service.start() only runs once)
3. Calls restoreMemoryPluginState() with the pre-register snapshot (which is empty of this plugin's supplement)

This ZAPS the previously registered supplement.

Fix

registerMemoryCorpusSupplement now checks if the plugin already has a registered supplement and skips adding a duplicate instead of replacing. This makes the registration idempotent: re-registering during lazy tool-load no longer wipes existing registrations.

Testing

• Plugin with MemoryCorpusSupplement registers inside service.start()
• Lazy tool-load triggers a second register() call
• Verify supplement is NOT lost after the re-register
• Verify memory_search corpus=wiki still returns results from the supplement

Fixes openclaw#77039