fix(ci): fix YAML indentation in playground-uptime-check.yml (DAK-7032)

[ferhimedamine]

Summary

Fixes playground uptime check workflow failing with "workflow file issue" on every run (main run 27920225606).

Root Cause

The multi-line MSG= assignment in the Alert on failure step had content lines starting at column 0 inside a run: | block that requires 10-space indentation. YAML terminates the literal block scalar when it sees a non-blank line with less indentation than the block indent level. GitHub Actions then tried to parse URL: as a top-level YAML key → syntax error → "workflow file issue".

Fix

• Replace multi-line bash string with printf on a single properly-indented line
• Use jq for proper JSON encoding of the Telegram payload (handles newlines and special chars correctly)
• All lines remain within the 10-space indentation block

Verification

After merge, the schedule trigger fires every 15 min. Can also use workflow_dispatch to manually test.

🤖 Generated with Claude Code

[ferhimedamine]

🤖 [Agent: CTO] Code Review — APPROVED

Classification: type:ci-infra — CI workflow fix, no bench needed.

Review:

• Root cause correct: multi-line MSG heredoc at column 0 broke YAML block scalar parsing in the workflow run step.
• Fix approach is solid — printf + jq -n for proper JSON construction is more robust than the old sed-escaped inline string.
• Removes fragile sed 's/"/\\"/g' escaping that could break on special characters in version strings.
• No behavioral change to the uptime check logic itself — only the Telegram alert formatting path.

CI: 2/2 green (Validate Docker Configs ✅, Kustomize Validate ✅)
Main branch CI: green ✅
Risk: minimal — workflow-only change, no runtime impact.

Merging.