github: say why a forge could not be reached instead of 'connection failed'

[MickLesk]

✍️ Description

The release lookup ran curl silently and reported every transport failure as 'Connection to github failed', exit 7. Reproduce the failure once with curl's message visible, classify the exit code (DNS, no route, timeout, TLS, reset), probe IPv4 against IPv6 for the common broken-v6 case, and return curl's real code so the error table matches.

🔗 Related Issue

Fixes community-scripts/ProxmoxVE#17835

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected (Proxmox VE and/or Incus, as applicable).
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.
• API.txt regenerated – If a function signature or behavior changed, API.txt was updated to match.

---

🤖 AI Assistance (X in brackets)

If you used an AI tool (GitHub Copilot, Claude, ChatGPT, etc.) to write or generate any code in this PR, you must confirm compliance below.
Select exactly one option.

• No AI used – Code was written without AI assistance.
• AI was used – The output has been reviewed and corrected, and I take responsibility for it matching this repo's conventions.

---

🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing behavior in a way that may require updates in every downstream script.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, docs/, CONTRIBUTING.md, or other docs.

[github-actions]

Try this branch

The engine and the scripts resolve independently, so a production script can
be run against the engine from this PR by setting one variable:

COMMUNITY_SCRIPTS_CORE_URL=https://raw.githubusercontent.com/community-scripts/core/fix/forge-connection-diagnosis \
bash -c "$(curl -fsSL https://raw.githubusercontent.com/community-scripts/ProxmoxVE/main/ct/debian.sh)"

Swap ct/debian.sh for whatever exercises the change.

Run a script from a fork as well

curl -fsSL https://raw.githubusercontent.com/community-scripts/core/fix/forge-connection-diagnosis/tools/run.sh |
  bash -s -- https://raw.githubusercontent.com/YOU/DevScripts/your-branch ct/debian.sh \
             https://raw.githubusercontent.com/community-scripts/core/fix/forge-connection-diagnosis

Note that run.sh is reached through a pipe, so the script it starts inherits
an exhausted stdin. Whiptail is fine — it opens /dev/tty — but a plain read
would see EOF. The single-variable form above does not have that problem.

Useful flags while testing

dev_mode=net logs every engine fetch with status and duration, which is the
quickest way to confirm the branch is really being used. dev_mode=keep stops a
failed build from deleting the container along with the evidence.