Preserve inner exceptions in RestService

[goal (yykkibbb)]

Summary

• Fixes Don't throw away the inner exception #1437
• Preserve inner exceptions in RestService.ExecuteOnOneInstanceAsync instead of discarding them
• Collect caught HttpRequestExceptions and pass them as innerException (single → direct, multiple → AggregateException)
• Pass the caught exception as innerException for the Unauthorized case

Test plan

• Added unit tests verifying inner exception preservation for single and multiple URLs
• All 85 existing unit tests pass

[goal (yykkibbb)]

Hi Matt Howlett (@mhowlett) Henrik Feldt (@haf) Dutchie (@dutchie031) torbjornsk,

I've submitted a PR to address the inner exception preservation issue discussed here. Would appreciate it if you could take a look when you have a chance.

Thank you!

[Robert Yokota (rayokota)]

/sem-approve

[Robert Yokota (rayokota)]

Thamks goal (@yykkibbb) , LGTM