cluster_link/replication: fix warn log message

[pgellert]

As per the title, fixes the source <-> shadow mixup in the warning log line, which made it misleading.

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
• v25.3.x
• v25.2.x
• v25.1.x

Release Notes

Improvements

• Improve cluster linking shadow topic prefix truncation error message.

[Copilot]

Pull request overview

This PR corrects misleading log messages in the cluster linking partition replication code. The warning and debug messages incorrectly referred to "source partition" and "truncated" when they should have specified "shadow partition" and "prefix truncated" respectively, which could confuse debugging efforts.

Changes:

• Fixed terminology in error log message from "source partition" to "shadow partition"
• Added "prefix" qualifier to clarify the type of truncation operation in both warning and debug messages

[vbotbuildovich]

CI test results

test results on build#79152

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
WriteCachingFailureInjectionE2ETest	test_crash_all	{"use_transactions": false}	integration	https://buildkite.com/redpanda/redpanda/builds/79152#019bc6ea-30a5-496a-80fc-3e485aeb376a	FLAKY	8/11	Test PASSES after retries.No significant increase in flaky rate(baseline=0.1040, p0=0.2794, reject_threshold=0.0100. adj_baseline=0.2807, p1=0.4359, trust_threshold=0.5000)	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=WriteCachingFailureInjectionE2ETest&test_method=test_crash_all

[vbotbuildovich]

/backport v25.3.x