This was implemented in rB14d56b4217f8: UI: Add angle snapping to line gesture tools but was never working for
tools/operators other than the sculpt line mask tool.

To be precise, the preview actually snapped but the operations (e.g.
mesh bisect, vertex weight gradient) still happened "unsnapped" in
modal. For the sculpt line mask tool this wasnt a problem, because it
only draws a preview while modal, the actual mask was only applied
later.

This solves part one of T91320 (snapping), sculpting also introduced
flipping in rB7ff6bfd1e0af: UI: Allow changing the active side of line gestures which does not make much sense for all
tools, but in bisect this could actually be supported, will add that in
a separate Diff.

ref T91320