ci: send Slack notifications on CI failures

[anakin87]

Related Issues

• part of https://github.com/deepset-ai/haystack-private/issues/256

Proposed Changes:

Instead of sending DD events for CI failures and then Slack notifications from DD monitors, skip DD

• create and publish a composite action (reusable across workflows and repos), based on slackapi/slack-github-action -> https://github.com/deepset-ai/notify-slack-action
• adopt the composite action in all CI-related workflows
• (skip for now the release workflow, to be handled in another PR)

To make this work, before opening this PR, I completed the setup described in the Slack documentation.

How did you test it?

In this PR, I temporarily made license_compliance workflow fail and got the expected notification.

Notes for the reviewer

From the safety point of view, we should be OK

• Of all Slack sending techniques, incoming webhooks are the least permissive method, with less surface area
• Passing secrets as input to composite action is safe and common: https://github.com/orgs/community/discussions/34212

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
haystack-docs	Ignored	Preview	Mar 6, 2026 6:56am

[coveralls]

Pull Request Test Coverage Report for Build 22752365539

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 28 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 92.768%

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Files with Coverage Reduction	New Missed Lines	%
core/pipeline/async_pipeline.py	28	71.18%

Totals
Change from base Build 22752247302:	0.0%
Covered Lines:	15610
Relevant Lines:	16827

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💛 - Coveralls