tests/rm_stm: wait for STM replay before LSO check after restart

[ballard26]

After restart_stm_and_raft() the STM has only applied through the snapshot offset; data batches are still asynchronously replaying, so last_stable_offset() can briefly return invalid_lso. Wrap the check in RPTEST_REQUIRE_EVENTUALLY, mirroring the wait already used at the end of the test.

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
• v26.1.x
• v25.3.x
• v25.2.x

Release Notes

• none

[Copilot]

Pull request overview

This PR stabilizes an rm_stm restart test by accounting for asynchronous log replay after restart_stm_and_raft(). It fits into the cluster transaction test suite by making the post-restart last_stable_offset() assertion wait until the STM has replayed far enough for LSO to become valid.

Changes:

• Replaces an immediate post-restart LSO check with RPTEST_REQUIRE_EVENTUALLY.
• Documents why last_stable_offset() can transiently return model::invalid_lso after restart.
• Keeps the existing semantic assertion that LSO remains held back by open transactions.

[vbotbuildovich]

Retry command for Build#83973

please wait until all jobs are finished before running the slash command

/ci-repeat 1
skip-redpanda-build
skip-units
skip-rebase
tests/rptest/tests/cluster_linking_e2e_test.py::ShadowLinkingReplicationTests.test_with_restart@{"storage_mode":"tiered_cloud"}

[vbotbuildovich]

CI test results

test results on build#83973

test_status	test_class	test_method	test_arguments	test_kind	job_url	passed	reason	test_history
FLAKY(FAIL)	ShadowLinkingReplicationTests	test_with_restart	{"storage_mode": "tiered_cloud"}	integration	https://buildkite.com/redpanda/redpanda/builds/83973#019deaa7-71d5-43cf-851d-c0c0e5ea6d2d	16/21	Test FAILS after retries.Significant increase in flaky rate(baseline=0.0123, p0=0.0001, reject_threshold=0.0100)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=ShadowLinkingReplicationTests&test_method=test_with_restart