[fix] Preserve then delimiter for when clauses with endless ranges (#880)

Ruby treats `..` at end-of-line as a line continuation operator, so
`when 0.9..` followed by a newline causes a SyntaxError. Emit `then`
after the conditions when the last condition is an endless range.

Fixes #878

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: thomasklemm

fixtures/small/when_endless_range_actual.rb

@@ -0,0 +1,38 @@
+case x
+when 0.9.. then "very strong"
+when 0.7.. then "strong"
+when 0.5.. then "moderate"
+end
+
+case x
+when 1.. then "positive"
+end
+
+case x
+when ..0 then "non-positive"
+end
+
+case x
+when 0..10 then "small"
+end
+
+case x
+when 0.9.., :special then "a"
+end
+
+case x
+when 0.9.. then
+  "very strong"
+end
+
+case x
+when 1... then "exclusive endless"
+end
+
+case x
+when :special, 0.9.. then "a"
+end
+
+case x
+when (0.9..) then "parenthesized endless"
+end

fixtures/small/when_endless_range_expected.rb

@@ -0,0 +1,48 @@
+case x
+when 0.9.. then
+  "very strong"
+when 0.7.. then
+  "strong"
+when 0.5.. then
+  "moderate"
+end
+
+case x
+when 1.. then
+  "positive"
+end
+
+case x
+when ..0
+  "non-positive"
+end
+
+case x
+when 0..10
+  "small"
+end
+
+case x
+when 0.9.., :special
+  "a"
+end
+
+case x
+when 0.9.. then
+  "very strong"
+end
+
+case x
+when 1... then
+  "exclusive endless"
+end
+
+case x
+when :special, 0.9.. then
+  "a"
+end
+
+case x
+when (0.9..)
+  "parenthesized endless"
+end

librubyfmt/src/format_prism.rs

@@ -4862,6 +4862,14 @@ fn format_when_node<'src>(ps: &mut ParserState<'src>, when_node: prism::WhenNode
         });
     });
 
+    // Ruby treats `..` at end-of-line as a line continuation operator.
+    // When the last condition is an endless range, we must emit `then`
+    // to prevent a SyntaxError.
+    if when_conditions_end_with_endless_range(&when_node) {
+        ps.emit_space();
+        ps.emit_keyword(b"then");
+    }
+
     ps.new_block(|ps| {
         ps.with_start_of_line(true, |ps| {
             ps.emit_newline();
@@ -4872,6 +4880,14 @@ fn format_when_node<'src>(ps: &mut ParserState<'src>, when_node: prism::WhenNode
     });
 }
 
+fn when_conditions_end_with_endless_range(when_node: &prism::WhenNode) -> bool {
+    when_node
+        .conditions()
+        .last()
+        .and_then(|c| c.as_range_node())
+        .is_some_and(|r| r.right().is_none())
+}
+
 fn format_while_node<'src>(ps: &mut ParserState<'src>, while_node: prism::WhileNode<'src>) {
     format_conditional_node(ps, b"while", true, &Conditional::While(while_node));
 }