Cherry-pick of #3945 to release-6.3

[yasserfaraazkhan]

Cherry pick of #3945 on release-6.3.

Restores per-OS GitHub commit statuses (e2e/linux, e2e/macos, e2e/windows, plus e2e/macos-policy / e2e/windows-policy on PR/master). CMT still keeps the e2e/compatibility-matrix-testing umbrella.

Also cherry-picks #3923 so required e2e/<os> checks can pass on this branch. release-6.3 still has the pre-11.10 MM-T4054 fixture; CMT on release-6.3 HEAD already fails MM-T4054 on server 11.10.0 only. Master is green because it already has #3923. This is a test-only change (e2e/specs/mattermost/media_preview.test.ts).

Summary

• Collapse TSIO history attempts per spec/job so a failed-then-retried test counts as one failure (or flaky if it later passed). Helpers live in e2e/utils/cmt-channel-notify.js (needed for per-OS aggregation).
• Prefer unique per-leg totals when flipping e2e/<os> checks from TSIO.
• Restore per-OS GitHub commit statuses for PR/master and CMT.
• Cherry-pick e2e: fix MM-T4054 media preview failure on Mattermost 11.10.0-rc2 #3923: wait for a loaded non-placeholder file-preview control before opening MM-T4054 (Mattermost 11.10 / MM-69174).

Ticket Link

Cherry-pick of #3945
Also includes #3923 (needed for required E2E checks vs server 11.10.0)

Checklist

• Added or updated unit tests (required for all new features)
• Has UI changes
• read and understood our Contributing Guidelines
• completed Mattermost Contributor Agreement
• executed npm run lint:js for proper code formatting
• Run E2E tests by adding label E2E/Run

Device Information

This PR was tested on: unit tests via node --test e2e/utils/cmt-channel-notify.test.js e2e/utils/tsio-report-status.test.js (40 passed)

Release Note

NONE

Backport notes

Automated cherry-pick conflicted because release-6.3 does not have channel-notify wiring from #3902.

What this PR includes:

• Per-OS pending + final commit statuses from ci(e2e): unique CMT failure counts + restore per-OS commit statuses #3945
• cmt-channel-notify.js helpers required to aggregate TSIO results by OS (parseCmtJobName, fetchPerJobCountsFromConsolidated, buildIndividualReportUrl)
• Unique-failure-count helpers and their unit tests
• e2e: fix MM-T4054 media preview failure on Mattermost 11.10.0-rc2 #3923 MM-T4054 media-preview test adaptation (required e2e/linux|macos|windows were red on the previous HEAD for a pre-existing 11.10.0 incompatibility, not because of ci(e2e): unique CMT failure counts + restore per-OS commit statuses #3945)

What this PR does not enable:

• Mattermost channel webhook posts (MATTERMOST_*_WEBHOOK_URL not wired on this branch; unique CMT headline counts only apply once channel notify is present)

Merge gate: after this push, re-add the E2E/Run label so Matterwick dispatches Playwright against the new HEAD (e733a294). Do not merge until e2e/linux, e2e/macos, and e2e/windows are successful on that SHA.

  

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