🛡️ Sentinel: [CRITICAL] Fix MITM vulnerability via InsecureSkipVerify

[rschumann]

🚨 Severity: CRITICAL
💡 Vulnerability: Found InsecureSkipVerify: true in http.Transport for testing proxies and discovering models. Hardcoding insecure TLS validation bypasses certificate verification, introducing a severe risk of Man-in-the-Middle (MITM) attacks.
🎯 Impact: An attacker could intercept and modify traffic between the gateway and external services, potentially stealing API keys or injecting malicious responses.
🔧 Fix: Removed InsecureSkipVerify: true from TLSClientConfig in both test_provider.go and discover_models.go to enforce standard TLS certificate validation. Also cleaned up unused crypto/tls imports.
✅ Verification: Verified by code inspection and running go test ./... -short.

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PR created automatically by Jules for task 17542999959663660292 started by @rschumann

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