ci: format CUOPT_SLACK_MENTION_ID as subteam when ID starts with S

[rgsl888prabhu]

Summary

• Detect user-group (subteam) IDs by their S prefix and emit <!subteam^S...> syntax.
• User IDs (prefix U/W) continue to use <@U...>.
• Update the contract comment to spell out both forms and warn that handle names alone will not ping.

Why

Today's nightly Slack post (2026-05-07) showed @cuopt-ci-team rendered as plain text — the message arrived but produced no notification. Root cause: ci/utils/generate_slack_payloads.py always wrapped CUOPT_SLACK_MENTION_ID as <@id>, which is user-only syntax. Slack requires <!subteam^SXXXXXXXX> to ping a user group. Passing the group's handle name (cuopt-ci-team) or its S... ID with the old formatter both fail silently.

Test plan

• CUOPT_SLACK_MENTION_ID=SXXXXXXXX → next nightly with new failures pings the cuopt-ci-team group.
• Sanity-check that a user ID (U...) still renders as <@U...> (no behavior change for that path).
• Empty/unset CUOPT_SLACK_MENTION_ID produces no mention (existing behavior preserved).

Notes

Builds on #1182, which plumbs the CUOPT_SLACK_MENTION_ID secret through the workflow. Either order of merge works; this PR only touches the formatter.

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📥 Commits

Reviewing files that changed from the base of the PR and between b38e56d and 1e55ba9.

📒 Files selected for processing (1)

• ci/utils/generate_slack_payloads.py

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📝 Walkthrough

Walkthrough

The PR updates ci/utils/generate_slack_payloads.py to conditionally format Slack mentions based on the CUOPT_SLACK_MENTION_ID environment variable. The logic now supports subteam mentions (IDs starting with S) using <!subteam^{id}> format, user mentions for other non-empty values using <@{id}> , and disables mentions when the value is empty.

Changes

Slack Mention Format Enhancement

Layer / File(s)	Summary
Conditional Mention Formatting ci/utils/generate_slack_payloads.py	Slack mention tag generation updated with conditional logic: <!subteam^{id}> for IDs starting with S, <@{id}> for other non-empty values, empty for blank values.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The pull request title directly and clearly summarizes the main change: updating Slack mention formatting to use subteam syntax when the ID starts with 'S'.
Description check	✅ Passed	The description is clearly related to the changeset, providing context about why the change is needed, what was changed, and how it will be tested.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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