🛡️ Sentinel: [CRITICAL] Fix missing SSRF protection in public proof bundle example

[mapleleaflatte03]

🚨 Severity: CRITICAL
💡 Vulnerability: The functions _fetch_live_manifest and _fetch_live_runtime_proof in examples/generate_public_proof_bundle.py used urllib.request.urlopen on URLs without validating the scheme. This left them vulnerable to SSRF (Server-Side Request Forgery) and arbitrary file reads (e.g., using file:///etc/passwd).
🎯 Impact: An attacker could potentially read internal files or access internal network services if they control the URL inputs to these scripts.
🔧 Fix: Added case-insensitive validation to ensure the URL scheme is strictly http:// or https:// before fetching. It returns a safe error dictionary otherwise.
✅ Verification: Verified by checking that urlopen in this file is now properly guarded, and running the kernel/tests suite to ensure no regressions.

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PR created automatically by Jules for task 6796407046001373342 started by @mapleleaflatte03

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[mapleleaflatte03]

This repository is now archived for historical reference.\n\nActive Meridian development has moved to the monorepo:\nhttps://github.com/mapleleaflatte03/meridian\n\nPlease open a new PR against the monorepo if this change is still needed.