🛡️ Sentinel: [HIGH] Fix MITM vulnerability in management handlers (InsecureSkipVerify)

[rschumann]

🚨 Severity: HIGH
💡 Vulnerability: The management handlers discover_models.go and test_provider.go used InsecureSkipVerify: true in their http.Transport configurations when connecting through proxies.
🎯 Impact: This disables TLS certificate validation, making the application highly susceptible to Man-in-the-Middle (MITM) attacks. An attacker could intercept and modify sensitive API requests and responses containing model data or API credentials.
🔧 Fix: Removed the TLSClientConfig with InsecureSkipVerify: true from the http.Transport configurations, enforcing standard TLS certificate validation. Cleaned up unused "crypto/tls" imports.
✅ Verification: Ran gosec to ensure the G402 vulnerabilities are resolved, and go test to confirm tests pass without regressions.

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PR created automatically by Jules for task 7847752988633958547 started by @rschumann

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