PLU-591: [show-multiple-rows-3]: add preprocess to postman email

[m0nggh]

TL;DR

Added support for table variables in Postman transactional email body with HTML table rendering and error handling.

What changed?

• Added preprocessVariable function to the send transactional email action that converts table variable markers to HTML tables
• Implemented type guard isTableMarker to identify table variable markers
• Restricted table variable usage to the email body field only, showing placeholder text for other fields
• Added graceful error handling that displays error messages instead of failing the entire step
• Updated the body field configuration to accept table variables alongside existing variable types
• Extended IFieldRichText interface to support variableTypes specification

How to test?

1. Create a workflow with a Postman send transactional email step
2. Use a table variable in the email body field and verify it renders as an HTML table
3. Try using a table variable in other fields (subject, recipient) and confirm placeholder text appears
4. Test with malformed table data to ensure error messages display properly instead of step failure
5. Verify existing variable types (text, array, etc.) continue to work as expected

Why make this change?

This enhancement allows users to include structured tabular data in their transactional emails, making it easier to send formatted reports, lists, or other structured information. The restriction to body-only usage and graceful error handling ensures the feature is both useful and robust.

[m0nggh]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• PLU-591: [show-multiple-rows-4]: add frontend table variable #1539
• PLU-591: [show-multiple-rows-3]: add preprocess to postman email #1538 👈 (View in Graphite)
• PLU-591: [show-multiple-rows-2]: modified step regex and add table variable marker #1537
• PLU-591: [show-multiple-rows-1]: add helper functions #1536
• develop-v2

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[linear]

PLU-591 Render find multiple rows in a table in Postman action