Fix match in if-then-else branches expanding to vertical with fit-or-vertical

[MisterDA]

When a match expr with appeared as a branch of if-then-else with if-then-else=fit-or-vertical, the match keyword, scrutinee, and with were broken onto separate lines even when they easily fit on one line:

if true then
  match
    expr
  with
  | true -> ()
  | false -> ()

The regression was introduced by 8056fa5 ("Overhaul begin match formatting"), which changed fmt_match to wrap pro inside a nested hvbox 0 (pro_inner $ keyword). When the match is inside an if-then-else branch, the pro carries a break_unless_newline 1000 2 (from the Fit_or_vertical branch_pro). This inflates the inner hvbox's size to 1000+, causing the Format engine to classify the parent hovbox as non-fitting, which forces all breaks in the match .. expr .. with box to break.

Fix: in match_inner_pro, return false for Pexp_ifthenelse context (same as already done for Pexp_infix), so that pro is placed outside the match's inner hvbox as pro_outer, where it does not interfere with the match keyword/expr/with fitting on one line.

Fix #2766. This fix was vibe-clauded.

[Julow]

That's the right fix, thanks !