[BUG] Using`$ now()` in Mapping throws an error

[vesnushka]

Describe the bug
We faced en error in extracting the questionnaire due to the use of $ now() in Mapping

Severity
Critical

Steps to reproduce the behavior:

1. The Mapping example with $ now()
   Note: the use of variables is required to reproduce the error

body:
  $let:
    patientId: >-
      $
      fhirpath("QuestionnaireResponse.repeat(item).where(linkId='patientId').answer.value.string").0
  $body:
    type: transaction
    entry:
      - request:
          url: /Observation
          method: POST
        resource:
          code:
            coding:
              - code: 29463-7
                system: http://loinc.org
                display: Body weight
          value:
            Quantity:
              unit: kg
              value: 80
          status: final
          subject:
            id: $ patientId
            resourceType: Patient
          effective:
            dateTime: $ now()
          resourceType: Observation
    resourceType: Bundle
id: test-now-extract
resourceType: Mapping

2. $debug returns correct result

POST /Mapping/test-now-extract/$debug
Content-Type: text/yaml

patientId: ede14f63-fffa-4ed0-aed8-b5b4f34f0055

Result:

type: transaction
entry:
  - request:
      url: /Observation
      method: POST
    resource:
      code:
        coding:
          - code: 29463-7
            system: http://loinc.org
            display: Body weight
      value:
        Quantity:
          unit: kg
          value: 80
      status: final
      effective:
        dateTime: 2023-03-20T14:35:59.726Z
      resourceType: Observation
resourceType: Bundle

3. But $apply returns the error

POST /Mapping/test-now-extract/$apply
Content-Type: text/yaml

patientId: ede14f63-fffa-4ed0-aed8-b5b4f34f0055

The answer

resourceType: OperationOutcome
...
text:
  status: generated
  div: >-
    Transaction failed at entry[0]. Response status is 422. Response body is
    {"resourceType":"OperationOutcome","text":{"status":"generated","div":"Invalid
    resource"},"issue":[{"severity":"fatal","code":"invalid","expression":["Observation.effective.dateTime"],"diagnostics":"expected
    type of string"}]}.
issue:
  - severity: fatal
    code: invalid
    diagnostics: expected type of string
    expression:
      - Observation.effective.dateTime

Expected behavior
$apply should return FHIR dateTime with no error

Versions:

• Aidbox image version: v:2302.b6d04231