fix: replace assert with error handling for stanza count mismatch

[sneurlax]

Replaces the panic-prone assert_eq!(stanzas.len(), expected_stanzas) with proper handling that:

1. Checks if the stanza count matches the expected count.
2. If there's a mismatch, creates a recipient::Error::Internal with a descriptive message.
3. Sends the error via the IPC channel using .send(&mut phase)?.
4. Returns gracefully instead of crashing.

Closes #576

[str4d]

NACK per #576 (comment) because I intentionally panic in the plugin helper code when MUST requirements are violated.