PLU-591: [show-multiple-rows-2]: modified step regex and add table variable marker

[m0nggh]

TL;DR

Added support for hex-encoded modifiers in step variables to enable table column selection functionality.

What changed?

• Extended the variable regex pattern to support optional hex-encoded modifiers in the format {{step.<uuid>.<path>|<hexModifier>}}
• Added parsing logic to decode hex-encoded modifiers and extract table column selection information
• Introduced TableVariableMarker interface to pass table data and selected column IDs to action preprocessors
• Added error handling for invalid hex encoding with appropriate logging

How to test?

1. Create a step variable with a hex-encoded table modifier: {{step.abc-123-def.data|7461626c653a636f6c31}}
2. Verify the modifier is correctly decoded (hex example decodes to "table:col1")
3. Test with FormSG table data to ensure JSON parsing works correctly
4. Verify that invalid hex encoding falls back gracefully and logs warnings
5. Confirm that variables without modifiers continue to work as before

Why make this change?

This enables dynamic table column selection in step variables, allowing users to specify which columns from table data should be processed by subsequent actions. The hex encoding approach keeps the variable syntax clean while supporting complex modifier parameters.

[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
• PLU-591: [show-multiple-rows-2]: modified step regex and add table variable marker #1537 👈 (View in Graphite)
• 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