Add an exception-mapping behavior

[kzu]

Summary

Ship a behavior that translates or wraps an exception from the target. Swallowing a failure is a separate, off-by-default option.

API

The map is Func<Exception, Exception?>. Null means leave the exception as it is. Any other instance replaces it. A separate switch, default off, lets the map turn a failure into a successful value.

Behavior

• Observe both failure channels through Proceed: a synchronous throw, and a faulted Task or ValueTask.
• A wrapped exception keeps the original as InnerException.
• A translated synchronous exception is still thrown by the stunt. A translated task fault remains a fault of the returned awaitable, with IMethodReturn.Exception null.
• Exceptions thrown by behaviors outside this one are left alone.

Acceptance

• An IOException from the target becomes the mapped exception, and the original is its inner exception.
• Awaiting a faulted Task<T> throws the mapped exception.
• With swallowing left off, the map cannot turn a failure into a value.