chore: cleanup Slack notifications — remove redundant titles, migrate release to Block Kit

[mateo-di]

Description

Shortcut

• Story: https://app.shortcut.com/cartoteam/story/538271
• Autolink: [sc-538271]

Cleanup Slack notifications across all CARTO workflow files. This is the litellm-fork counterpart to CartoDB/cloud-native#23436.

Changes

1. Remove redundant title (4 notifications)

All Block Kit notifications had a top-level "text" field alongside "attachments". Slack renders "text" as plain text ABOVE the colored sidebar, duplicating the header inside the block. Fixed by removing "text" and using the attachment-level "fallback" field (notification preview only, not rendered in channel).

Affected files:

• carto-upstream-sync-main.yml — upstream sync status (Steps 1-2)
• carto-upstream-sync-ready-checker.yml — tests passed (Step 4)
• carto-slack-changelog.yml — PR merge + release changelog

2. Conciseness improvements

• Removed header blocks, replaced with bold mrkdwn in section blocks
• Removed divider blocks and redundant context blocks
• Folded title/type info into single section line

3. Migrate carto-release.yaml to Block Kit

The release notification was the last remaining old-format notification using plain text via -F "text=..." with Unicode box-drawing separators. Migrated to Block Kit with:

• Colored sidebar (blue for upstream sync, green for feature, orange for patch)
• Structured fields (CARTO Version, Upstream Base)
• Action buttons (View Release, View Logs)
• JSON validation before sending
• Switched from form-encoded to JSON POST

Type of change

• Chore

Acceptance

1. Create a CARTO release and verify the Slack notification in #cartodb-ops uses Block Kit with colored sidebar instead of plain text with Unicode separators
2. Trigger an upstream sync and verify notifications in #litellm-fork have no redundant title above the colored block
3. Merge a PR to carto/main and verify the changelog notification has no redundant title

Basic checklist

• Good PR name
• Shortcut link
• Just one issue per PR
• GitHub labels
• Proper status & reviewers
• Tests
• Documentation