fix: remove GHCR image checks from release availability CI (only verify GitHub release exists)

[Copilot]

Summary

Fixes the failing Integration Release Availability CI job.

Root Cause

The CI job was checking two things for each dependency:

1. ✅ GitHub release existence (gh release view) — this passed
2. ❌ GHCR image manifest (docker manifest inspect) — this failed for gh-aw-firewall:v0.25.3

Reference: https://github.com/github/gh-aw/actions/runs/23707131893/job/69060824275#step:5:1

Fix

As requested, simplified the release checkers to only verify the GitHub release exists. Removed:

• GHCR image manifest checks from Check gh-aw-firewall release
• GHCR image manifest checks from Check gh-aw-mcpg release
• GHCR image manifest checks from Check github-mcp-server release
• The now-unused "Define helpers" step that created the Python script for GHCR digest parsing

[Copilot]

Pull request overview

This PR fixes the failing Integration Release Availability CI job by simplifying dependency availability checks to verify only that the corresponding GitHub Release exists (removing GHCR image manifest inspection that was causing false failures).

Changes:

• Removed GHCR image manifest inspection (docker manifest inspect) from multiple dependency release-check steps.
• Deleted the now-unused helper step that generated a Python script for parsing GHCR digests.
• Kept GitHub release existence checks (gh release view) as the sole validation mechanism.

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