🛡️ Sentinel: Enforce payload size limit on mod pack imports

[TheDragonSoft]

🚨 Severity: MEDIUM

💡 Vulnerability: Uncontrolled resource allocation / memory exhaustion DoS vulnerability during mod pack import.

🎯 Impact: An oversized payload string passed to pack import could cause high memory consumption and application crashes.

🔧 Fix: Added a 2 MB size limit (MAX_PACK_CODE_LEN) in import_pack and import_pack_base64 before parsing JSON or decoding Base64 payloads.

✅ Verification: Added unit test base64_import_fails_on_oversized_input verifying oversized payloads are safely rejected.

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PR created automatically by Jules for task 2577023773017839190 started by @TheDragonSoft

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