Add echo endpoint to server for sync check

[paulgb]

This is extracted from #353 and includes only the server-side change.

It adds a custom message type that simply echoes back the content it received. Since the server processes messages sequentially, this is a way for a client to confirm that a message has been processed:

• send some message
• send a sync status message with a nonce
• when you have received a message with the same nonce back, you can assume the message has been processed

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[ellipsis-dev]

👍 Looks good to me! Reviewed everything up to 490c304 in 1 minute and 31 seconds

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• Looked at 24 lines of code in 1 files
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1. crates/y-sweet-core/src/doc_connection.rs:186

• Draft comment:
  Using OnceLock::set can panic if called more than once. Consider using OnceLock::get_or_init or checking if it's already set to avoid potential panics.
• Reason this comment was not posted:
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