Clamp "Messages left to delete" so it never goes negative

[biggdawg320]

While a large group delete is running, ServiceControl computes "messages left" as the starting count minus the deleted count, each from a separate query. If new failures keep arriving in the group during the delete, the deleted count can overtake the starting count, and the frontend rendered that negative value directly. This change clamps the displayed remaining count to a minimum of zero in MessageGroupList.vue's delete-progress panel, so the counter no longer overflows below zero while a delete is in progress. Added a regression test that mounts the group list with a group mid-delete whose remaining count is negative and asserts the UI shows 0 instead.

Fixes #3126

Testing

• Added a test (src/Frontend/src/components/failedmessages/messageGroupDeleteCounters.spec.ts) that fails on the current code and passes with this change.
• The surrounding tests pass: cd src/Frontend && npx vitest run ./src/stores ./src/components/failedmessages

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This change was drafted with AI assistance (Claude) and verified against the project's own test suite before submission.

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[rbev]

Thanks for the contribution, we will not be merging this pull request as it will be fixed in ServiceControl via Particular/ServiceControl#5939

[rbev]

This issue was found to only be visible in ServicePulse when using the upcoming SQL storage engines in ServiceControl.
It was not able to be replicated on the current RavenDB storage engine.