Preserve operation interrupted in ContextEval error

[dims]

When ContextEval detects an InterruptError, wrap both the original error and the context cause using fmt.Errorf("%w: %w", ...) so that:

• errors.Is(err, context.Canceled) works (new behavior)
• errors.Is(err, context.DeadlineExceeded) works (new behavior)
• errors.Is(err, InterruptError{}) works (preserved)
• strings.Contains(err.Error(), "operation interrupted") works (backward compat)

Previously, the error was replaced entirely with context.Cause(ctx), which dropped the "operation interrupted" message that downstream consumers (e.g. Kubernetes) rely on for string matching.

Fixes #1195 while maintaining backward compatibility.

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[dims]

xref: kubernetes/kubernetes#138334

[TristonianJones]

/gcbrun