fix: make the non-merge path in Execute() exception-safe [patch]

[matt-edmondson]

Fixes #70.

UndoRedoService.Execute(ICommand) cleared forward history and cleaned up save boundaries before calling command.Execute():

_stackManager.ClearForward();
_saveBoundaryManager.CleanupInvalidBoundaries(_stackManager.CurrentPosition);

command.Execute();

_stackManager.AddCommand(command);

When command.Execute() threw, the exception propagated correctly — but the pre-existing redo history, and any save boundaries inside it, had already been permanently discarded for a command that was never applied.

The comment on the auto-merge branch just above claimed this path already mirrored its exception safety ("This mirrors the non-merge path below"). It didn't; the non-merge path was evidently missed when the merge path and CompositeCommand were hardened.

The change

command.Execute() moves ahead of the two destructive calls. Nothing else moves.

Behaviour on success is identical, for two reasons worth stating because they are what makes the reorder safe rather than merely later:

• CurrentPosition is not touched by command.Execute(), so CleanupInvalidBoundaries() still receives exactly the position it received before.
• StackManager.AddCommand() already removes anything after CurrentPosition itself when branching, so the explicit ClearForward() was never what made the stack correct — it only mattered for the boundary cleanup's position argument, which is unchanged.

Tests

Two tests added next to the existing exception-safety tests, both verified to fail before the change and pass after it (by stashing the UndoRedoService.cs change and re-running):

Test	Fails without the fix with
Execute_CommandThrowsException_PreservesRedoHistory	CommandCount is 1, not 3 — B and C silently lost
Execute_CommandThrowsException_PreservesSaveBoundariesInForwardHistory	SaveBoundaries.Count is 0, not 1

The first also asserts the preserved history is still replayable — both Redo() calls reapply B and C — rather than only that the count survived. The existing Execute_CommandThrowsException_DoesNotCorruptStack missed this bug because it exercises a stack with no redo history to lose.

Full suite: 48 passed, 0 failed, 0 skipped.

🤖 Generated with Claude Code

https://claude.ai/code/session_01T1ntzPwg732TrH7D5vWpSV

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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