[release/6.0] Fix Enum.Parse's parse failure exception to include value in error message

[github-actions]

Backport of #57945 to release/6.0

/cc @stephentoub

Customer Impact

Exception message coming from Enum.Parse isn't as helpful as it previously was (and looks wrong, with '' in the message where there's obviously supposed to be a value).

Testing

Added new test for CI

Risk

Minimal change, low risk on an exception path. Primary concern would be if it somehow regressed perf, but I didn't measure any impact locally.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[stephentoub]

cc: @danmoseley

[danmoseley]

approved for 6.0 -- significant regression if you're debugging, low risk localized change.