Add visible warning when label addition fails during issue escalation

[Copilot]

Addresses feedback on #144 about silent label addition failures. When the bot escalates an issue but can't add the needs-maintainer label, users and maintainers had no visibility into the failure.

Changes:

• Move label addition before comment posting to enable failure detection
• Track label addition failures with labelAdditionFailed flag
• Append visible warning to escalation comments when labels can't be added

Example output when label addition fails:

<details><summary>🏷️ Escalated to maintainers</summary>

A maintainer will review this issue. No further bot responses will be sent.

⚠️ **Note:** Unable to automatically add the `needs-maintainer` label. Please add it manually to ensure proper triage.

[Wiki](https://github.com/microsoft/security-devops-action/wiki) · [FAQ](https://github.com/microsoft/security-devops-action/wiki/FAQ)
</details>

Workflow continues gracefully on label API failures while ensuring visibility for manual triage.

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