Cherry-pick of #3945 to release-6.2

[yasserfaraazkhan]

Cherry pick of #3945 on release-6.2.

Restores per-OS GitHub commit statuses (e2e/linux, e2e/macos, e2e/windows) for CMT, alongside the existing e2e/compatibility-matrix-testing umbrella. PR/master E2E already posted per-OS checks on this branch; those now use canonical linux|macos|windows names.

Summary

• Normalize instance_details platforms to linux|macos|windows (and pin macos-latest → macos-26) so e2e/<os> statuses stay consistent.
• Post pending e2e/linux|macos|windows on CMT start, then flip them from CMT matrix job conclusions (this branch has no TSIO reporter).
• Keep the CMT umbrella check e2e/compatibility-matrix-testing.
• Restructure the 6.2-only status helpers so no-nested-ternary / no-negated-condition pass (cmtOsCommitStatus, playwrightProjectForOs).

Ticket Link

Cherry-pick of #3945

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/github-actions.test.js (10 passed); npx eslint e2e/utils/github-actions.js e2e/utils/github-actions.test.js --quiet clean

Release Note

NONE

Backport notes

Automated cherry-pick conflicted because release-6.2 does not have TSIO reporting (tsio-report-status.js) or channel notify (cmt-channel-notify.js).

What this PR includes:

• Per-OS CMT commit statuses from ci(e2e): unique CMT failure counts + restore per-OS commit statuses #3945, implemented without TSIO by aggregating CMT job names (linux-11.9.0, …)
• Platform canonicalization for PR E2E statuses (already per-OS on this branch)

What this PR does not include:

• Unique CMT channel-notify failure counts (no Mattermost webhook notify on this branch)
• TSIO per-leg report links / e2e/<os>-policy status rename (policy-test/<os> remains)
• e2e: fix MM-T4054 media preview failure on Mattermost 11.10.0-rc2 #3923 / media_preview.test.ts — that spec does not exist on release-6.2

Merge gate: wait for ci (build-linux, build-mac-no-dmg, build-win-no-installer, report-test-results) on HEAD 393ae8bb. Playwright E2E/Run is not required for this CI-only change; latest release-6.2 CMT is already green. Needs a review (currently none).

  

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❌ E2E Test Setup Failed

Failed to create E2E test instances: failed to create installation: failed with status code 409