Improve secret detection with caching and setter support

[somiljain2006]

Optimize secret detection and add ClassValue cache

• Introduce a ClassValue-backed cache to avoid repeated reflection, improving performance and preventing ClassLoader memory leaks.
• Enhance secret detection by checking for setters that accept hudson.util.Secret.
• Make calculateIfSecret return a primitive boolean (defaulting to false) to ensure safe handling without nullable states.
• Reduce log noise by removing repeated logging for null targetClass cases.

Add tests covering:

• getter, field, and setter-based secret detection
• inheritance scenarios
• non-secret cases
• cache consistency (idempotency)

Manual verification performed:

• confirmed cache effectiveness via logging
• verified secrets are masked in exported YAML
• validated no ClassLoader leaks via plugin install/uninstall cycle

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[timja]

looks fine