Fix continuation action sorting in Fix Common Errors

[mjuhasz]

Summary

This PR changes the Fix Common Errors action list so continuation-style fixes sort under a single visible action label while still keeping the two fix variants distinct internally.

Previously, FixContinuationStyle used a trailing-space hack in the action text to distinguish the current-line suffix fix from the next-line prefix fix. That preserved selection/apply behavior, but it also caused sorting by Action to split the rows into two visually identical groups.

In the screenshot below, the grid is sorted by Action, but the numbering restarts midway without any clear reason. This inconsistency prompted my investigation.

What changed

• Added an internal FixActionKey helper to separate:
  • the internal action identity
  • the user-visible action label
• Updated FixContinuationStyle to use explicit internal keys for:
  • suffix/current-paragraph fixes
  • prefix/next-paragraph fixes
• Updated FixDisplayItem to expose:
  • Action as the internal key
  • ActionDisplay as the normalized visible label
• Updated the Fix Common Errors grid to bind the Action column to ActionDisplay
• Added a regression test covering the action-key/display split

Result

• Sorting by Action no longer creates two visually identical continuation-style groups
• Continuation-style fix rows still remain distinct for selection persistence and apply behavior
• No user-facing label changes were introduced

Alternative solution

This would have worked too: give the two continuation rows genuinely different action names, such as:

• Fix continuation style: Custom (suffix)
• Fix continuation style: Custom (prefix)

or localized equivalents, the same way src/libse/Forms/FixCommonErrors/FixEmptyLines.cs:20 uses distinct labels for top/bottom/middle.

I did not choose that approach because it changes the UI semantics:

• it exposes an implementation detail the user may not care about
• it would sort into separate visible groups by design
• it would require new user-facing/localized strings if done cleanly

The action-key/display split keeps the UI behavior as I expected it:

• one visible action group: Fix continuation style: Custom
• two internally distinct rows, so selection/apply still works correctly

So a solution similar to RemovedEmptyLineAtTop pattern was a valid option, but it would have solved the identity problem by making the labels visibly different, whereas this fix solves it without changing the user-facing label.