Fix: 'contract' relationship not populated on emitted events

[devanshi00]

Summary

org.accordproject.cicero.runtime.Obligation declares a required relationship back to the governing contract:

abstract event Obligation {
  --> Contract contract
  --> Participant promisor optional
  --> Participant promisee optional
  o DateTime deadline optional
}

Any event emitted by a template needs to extend Obligation (e.g. PaymentObligationEvent), thereby inheriting this required contract field, causing:

ValidationException: The instance "<namespace>.<EventType>#<uuid>" is missing the required field "contract".

Changes Made

Based on @mttrbrts opinion

• Added populateObligationBackReferences(events, data) in TemplateArchiveProcessor.trigger(), invoked immediately before the existing event-validation loop.
• For each emitted event: skip if contract is already set (template logic can still set it explicitly)
• If the event extends Obligation and contract is unset, and data (the contract/clause instance passed into trigger()) itself extends Contract, look up data's identifying field via modelManager.getType(data.$class).getIdentifierFieldName() and build the relationship as a "<identifierValue>" string — the correct serialized form for a relationship field than assigning the resource itself.
• If data does not extend Contract, or its identifier can't be resolved, throw a clear error naming the offending event instead of letting the opaque ValidationException surface from resourcevalidator.js.
• Added RUNTIME_OBLIGATION_FQN (org.accordproject.runtime@0.2.0.Obligation) and RUNTIME_CONTRACT_FQN (org.accordproject.contract@0.2.0.Contract) constants to utils.ts.

Fixes bug

#170

@mttrbrts @dselman @DianaLease