ci: fix test notifications for weekly/release tests

[balasaajay]

• I, the PR author, have personally reviewed every line of this PR.

What does this PR do?

Enhance test notifications for weekly/release tests into a single root notification for MR, nightly, weekly, and release pipelines. Update tests to ensure notifications are correctly delegated to the triage package without direct alerts from individual workload runners.

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[ko3n1g]

/review

[svcnemo-autobot]

🤖 On it — handling this as review.

[svcnemo-autobot]

🔎 On it — resolving the pull request and reviewing it rigorously.

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🤖 I couldn't get started on this yet — tell me a bit more (which repository, and what change) and I'll pick it up.

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/30426964001

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/30500754684

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/30502503886

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/30502652199