[servicenow] Add Parsing for ECS Timestamp Field

[mohitjha-elastic]

Proposed commit message

servicenow: Add explicit date parsing for ECS @timestamp field.

Added explicit date parsing logic for the ECS @timestamp field to ensure consistent and accurate
timestamp handling during ingestion. Additionally, this ensures that the timestamp is explicitly parsed 
even when other date fields are not parsed, helping to avoid potential ingestion errors.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

How to test this PR locally

• Clone integrations repo.
• Install the elastic package locally.
• Start the elastic stack using the elastic package.
• Move to integrations/packages/servicenow directory.
• Run the following command to run tests.

elastic-package test -v

Related Issues

• Closes [ServiceNow]: @timestamp field isn't being formatted and causing a "date_time_parse_exception" #16514

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[elastic-vault-github-plugin-prod]

🚀 Benchmarks report

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[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 93a8667

cc @mohitjha-elastic

[ShourieG]

My concern here is with formats like

• MM-dd-yyyy
• dd-MM-yyyy

as these are ambiguous from a global timestamp perspective. In india it's common trend to use dd-MM-yyyy or dd/MM/yyyy or dd.MM.yyyy. But in many places we use mm-dd-yyyy and etc.

How do we distinguish if the date 01-02-2026 or 05-07-2026 is in MM-dd-yyyy format or dd-MM-yyyy format ? Do we expect '@timestamp' to be normalised in any way ? If not how can we decide upon the right order of execution here ? If '@timestamp' is normalised as month-day-year, why have support for both patterns ?