sl: Fix error code on failing over non shadowed topic

[michael-redpanda]

Was originally returning 409. Now returning 404.

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
• v24.3.x

Release Notes

Bug Fixes

• Attempt to fail over a topic that isn't being Shadowed now returns a 404 rather than a 409

[Copilot]

Pull request overview

This PR fixes an error code bug in the shadow link service where attempting to fail over a non-shadowed topic incorrectly returned HTTP 409 (Conflict/Already Exists) instead of HTTP 404 (Not Found).

Key Changes:

• Moved cluster_link::errc::topic_not_being_mirrored from the "already exists" (409) error case to the "not found" (404) error case

[vbotbuildovich]

CI test results

test results on build#76963

test_class	test_method	test_arguments	test_kind	job_url	test_status	passed	reason	test_history
MultiRestartTest	test_recovery_after_multiple_restarts	{"cloud_storage_type": 2}	integration	https://buildkite.com/redpanda/redpanda/builds/76963#019abbde-172c-4fc8-be49-1a17d791548d	FLAKY	20/21	upstream reliability is '94.4954128440367'. current run reliability is '95.23809523809523'. drift is -0.74268 and the allowed drift is set to 50. The test should PASS	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=MultiRestartTest&test_method=test_recovery_after_multiple_restarts
TieredStorageTest	test_tiered_storage	{"cloud_storage_type_and_url_style": [1, "path"], "test_case": {"name": "(TS_Read == True, TS_Timequery == True, SpilloverManifestUploaded == True)"}}	integration	https://buildkite.com/redpanda/redpanda/builds/76963#019abbca-8b36-4415-8f7e-775d42b851df	FLAKY	20/21	upstream reliability is '99.32432432432432'. current run reliability is '95.23809523809523'. drift is 4.08623 and the allowed drift is set to 50. The test should PASS	https://redpanda.metabaseapp.com/dashboard/87-tests?tab=142-dt-individual-test-history&test_class=TieredStorageTest&test_method=test_tiered_storage

[vbotbuildovich]

/backport v25.3.x