test(canton): unit-test sign event verification failures

[Pessina]

Follow-up from PR #740.

Code:

mpc/chain-signatures/node/src/indexer_canton/stream.rs

Lines 407 to 419 in 93c8e00

	/// Verify a SignBidirectionalEvent before processing it.
	///
	/// These checks are defense-in-depth on top of the Daml ledger guarantees:
	/// 1. Operators from the payload must be actual signatories on the CreatedEvent
	/// 2. Requester must be a signatory
	/// 3. An ExercisedEvent with choice "SignBidirectional" on Signer:Signer must
	/// exist in the same transaction — proves the event was created through the
	/// correct Daml code path, not fabricated
	///
	/// TODO(test): unit test each check in isolation — craft events where one
	/// check fails and verify the correct error is returned. Test: non-signatory
	/// operator, non-signatory requester, missing ExercisedEvent.
	fn verify_sign_event(

Context: verify_sign_event is defense-in-depth on top of Daml ledger guarantees. Add isolated unit tests for each failure case: non-signatory operator, non-signatory requester, and missing SignBidirectional ExercisedEvent on the configured Signer contract.