Add strict export mode to propagate export failures as exceptions

[somiljain2006]

Introduce a strict export mode in ConfigurationAsCode that propagates export failures as ConfiguratorException instead of embedding "FAILED TO EXPORT" markers in the generated YAML.

• Add export(OutputStream, boolean strict) API
• Preserve existing behavior by default (non-strict mode)
• Update Attribute.describe() to throw in strict mode
• Add unit tests covering strict and non-strict behavior
• Add API test to verify strict export on a valid Jenkins instance

This improves error visibility while maintaining backward compatibility.

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[timja]

why do you need this in the global config? just get it from the context.

[somiljain2006]

I initially added strictExport to CasCGlobalConfig to allow configuring it via YAML, but that effectively makes it a persistent server setting.
Based on your suggestion, I plan to completely remove it from the global config (which means intentionally dropping the YAML configurability) and overload the export method to accept a ConfigurationContext directly.
This decouples the export logic from the global database. Let me know if you agree with this, and I’ll go ahead and update the implementation accordingly.