model/x: Fix typo in legacy offset_options envelope template

[oleiman]

s/transform_offset_options/legacy_transform_offset_options/

Also adds unit tests to try prove out the theory of selectively writing out older versions of the serde::variant. Works as expected.

Also worth noting that this works regardless of the serde::envelope CRTP param fixed by this diff...perhaps that's due to the narrow scope of the legacy struct - only ever being written out explicitly in transform_metadata::serde_write.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v24.1.x
• v23.3.x
• v23.2.x

Release Notes

• none

[oleiman]

CI Failures:

• CI Failure (Data never drops below 1MB) in FullDiskReclaimTest.test_full_disk_triggers_gc #18014 (known & unrelated)
• CI Failure (AssertionError: Transformed partitions must the same or larger watermark) in TxAtomicProduceConsumeTest.test_basic_tx_consumer_transform_produce #20831 (new but this diff contains no functional changes outside unit tests)
  • ^^ closed as a dup of this one but seemingly still failing
• CI Failure (key symptom) in MaintenanceTest.test_maintenance_sticky  #20999 (known & unrelated)
• CI Failure (Failed to initialize Producer ID: Broker: Not coordinator) in TxUpgradeTest. upgrade_does_not_change_tx_coordinator_assignment_test  #19954 (known & unrelated)
• CI Failure (Unexpected tx_error error: {tx_errc::stale}) in TxAdminTest.test_mark_transaction_expired #19322 (known & unrelated)

[oleiman]

/ci-repeat 1

[oleiman]

/dt

[vbotbuildovich]

skipped ducktape retry in https://buildkite.com/redpanda/redpanda/builds/51121#0190801a-73c9-4bdc-af4e-cd7f8ff1da6c:
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skipped ducktape retry in https://buildkite.com/redpanda/redpanda/builds/51121#01908004-0761-4f7d-b5b2-d3cdf8eb6421:
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