[BUG] Date processor does not always include timezone

Describe the bug

The date processor does not always produce the correct format string as output. I can see this happens when the date is from a previous year. But, I have not yet looked to see if other situations could cause this.

To Reproduce

1. Create a pipeline with a date processor similar to the following:

    - date:
        match:
          - key: "calculated_at"
            patterns: [ "YYYY-MM-dd" ]
        destination: "calculated_at"
        output_format: "yyyy-MM-ddXXX"
        source_timezone: "UTC"
        destination_timezone: "UTC"
        locale: "en_US"

2. Run the pipeline.

3. Ingest data with the current year and last year:

curl http://localhost:2021/log/ingest -X POST -H 'Content-Type: application/json' -d '[{"calculated_at": "2024-12-31"},{"calculated_at": "2025-01-01"}]'

See the incorrect results:

data-prepper           | {"id":"year2024","calculated_at":"2024-12-31","calculated_at_text":"2024-12-31"}
data-prepper           | {"id":"year2025","calculated_at":"2025-01-01Z","calculated_at_text":"2025-01-01Z"}

Expected behavior

The date from 2024 should include the Z in the format.

Screenshots

N/A

Environment (please complete the following information):

Data Prepper 2.10.2

Additional context

This also prevents ingestion when using an upsert and a document Id. Interestingly, OpenSearch will accept the data when using insert.

Full pipeline sample:

date-test:
  delay: 10
  source:
    http:

  processor:
    # curl http://localhost:2021/log/ingest -X POST -H 'Content-Type: application/json' -d '[{"calculated_at": "2024-12-31"},{"calculated_at": "2025-01-01"}]'
    - date:
        match:
          - key: "calculated_at"
            patterns: [ "YYYY-MM-dd" ]
        destination: "calculated_at"
        output_format: "yyyy-MM-ddXXX"
        source_timezone: "UTC"
        destination_timezone: "UTC"
        locale: "en_US"
#    - add_entries:
#        entries:
#          - key: calculated_at
#            format: '${calculated_at}Z'
#            overwrite_if_key_exists: true
#            add_when: length(/calculated_at) == 10
    - copy_values:
        entries:
          - from_key: calculated_at
            to_key: calculated_at_text

  sink:
    - opensearch:
        hosts: [ "https://opensearch:9200" ]
        insecure: true
        username: admin
        password: myStrongPassword123!
        document_id: "${/id}"
        action: upsert
        index: test_date_with_timezone
        flush_timeout: -1
        template_type: index-template
        template_content: >
          {
            "template" : {
              "mappings" : {
                "properties" : {
                  "calculated_at" : {
                    "type" : "date",
                    "format": "yyyy-MM-ddXXX"
                  },
                  "calculated_at_text" : {
                    "type" : "text"
                  }
                }
              }
            }
          }
    - stdout:

Start by locating the date processor implementation and its tests, then reproduce the pipeline with the two dates shown in the issue. The fix is done when both outputs include the UTC Z suffix and the documented upsert ingestion succeeds without the timezone-related format mismatch.