Deduplicate Slack CI notifications with artifact-based state tracking

[potiuk]

Slack notifications for CI failures and missing doc inventories were posted on every
failing run regardless of whether the failure was already reported. This adds per-branch
state tracking via GitHub Actions artifacts so notifications are only sent when the set
of failures changes or 24 hours pass (as a "still not fixed" reminder). Green recovery
notifications are posted when a previously-failing run passes.

Changes:

• New scripts/ci/slack_notification_state.py — shared state management script that
  downloads previous state from artifacts, compares with current failures, and outputs
  one of: notify_new, notify_reminder, notify_recovery, or skip
• ci-image-checks.yml — inventory notifications now stateful with 3 message types
  (new, reminder, recovery) and deep-linked to the specific docs build job
• ci-amd-arm.yml — notify-slack-failure → notify-slack, runs on both success
  and failure (for recovery), includes failing job names in messages
• ci-notification.yml — external branch monitoring now stateful with 3 message types
• workflow_status.py — extended to output run ID and failing job names

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Opus 4.6 (1M context)

Generated-by: Claude Opus 4.6 (1M context) following the guidelines

[jscheffl]

Cool

[github-actions]

Backport failed to create: v3-1-test. View the failure log Run details

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
❌	v3-1-test

You can attempt to backport this manually by running:

cherry_picker 60e4393 v3-1-test

This should apply the commit to the v3-1-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

If you don't have cherry-picker installed, see the installation guide.