Improve proofreading tests (unit-e2e coverage + Playwright safety net)

[MichaelBuessemeyer]

Background

Follow-up from PR #9699 (thread). The new tests in proofreading_split_mapping.spec.ts thoroughly cover the return values of splitAgglomerateInMapping, but they don't verify the end-to-end behavior when a foreign proofreading update action is polled in: that the correct segment ids (not agglomerate ids) are requested for reload and that meshes are refreshed correctly. This is the bug class fixed in 587dde5.

Unit-e2e tests ("e2e" without a real backend)

Extend our existing proofreading test setup (BackendMock + AgglomerateMapping, as in proofreading_single_user/proofreading_multi_user) to cover, with high control:

• An injected foreign splitAgglomerate update action is correctly handled
• The correct segment ids are requested to be reloaded (and not agglomerate ids) — regression guard for the Keep mesh opacity after proofreading action mesh refreshing #9699 bug
• Meshes are correctly reloaded (and the right meshes removed) after the split
• Mesh opacity is preserved across the refresh (the actual fix shipped in Keep mesh opacity after proofreading action mesh refreshing #9699)

Test-infra cleanup

• Migrate proofreading_split_mapping.spec.ts off the hand-written mapping fixtures (error-prone) and onto BackendMock / AgglomerateMapping
• Once migrated, remove the bespoke mapping fixtures

Playwright (additional safety net)

• Emulate realistic user sessions in a predictable manner with end-of-test sanity checks (e.g. merge everything into one segment), optionally covering mesh handling — explicitly in addition to, not a replacement for, the unit-e2e tests above