Make ConditionBaseAttribute.Mode public

[Youssef1313]

Fixes #5581

[Youssef1313]

/backport to rel/3.9

[github-actions]

Started backporting to rel/3.9: https://github.com/microsoft/testfx/actions/runs/15131736276

[github-actions]

@Youssef1313 backporting to rel/3.9 failed, the patch most likely resulted in conflicts:

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: Make ConditionBaseAttribute.Mode public
Applying: Public
Using index info to reconstruct a base tree...
M	src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt
Falling back to patching base and 3-way merge...
Auto-merging src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt
CONFLICT (content): Merge conflict in src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0002 Public
Error: The process '/usr/bin/git' failed with exit code 128

NOTE: A PR will be created, but needs to be revised manually!