🛡️ Sentinel: [CRITICAL] Fix Path Traversal in Auth Management Handlers

[rschumann]

🚨 Severity: CRITICAL
💡 Vulnerability: Path Traversal via name query parameter in UploadAuthFile and DeleteAuthFile.
🎯 Impact: An attacker could potentially upload an auth file outside the auths directory or delete arbitrary files on the system if switchAILocal is running on Linux but targeted with a Windows-style path payload (\).
🔧 Fix: Changed the path traversal check to use strings.ContainsAny(name, "/\\") explicitly instead of os.PathSeparator.
✅ Verification: Verify using go test ./internal/api/handlers/management -run TestUploadAuthFile_PathTraversal and TestDeleteAuthFile_PathTraversal.

Reused a mock coreauth.Store implementation to simulate auth manager environments effectively during tests, improving their reliability.

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PR created automatically by Jules for task 10945029822529610499 started by @rschumann

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