[formatter] Fix missing blank lines before decorated classes in .pyi files (#18888)

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_python_formatter/resources/test/fixtures/ruff/stub_files/decorated_class_after_function.pyi

@@ -0,0 +1,9 @@
+# Issue #18865: Decorated classes below functions should be separated with blank lines
+def hello(): ...
+@lambda _, /: _
+class A: ...
+
+def world(): ...
+
+@final
+class B: ...

crates/ruff_python_formatter/src/preview.rs

@@ -20,3 +20,10 @@ pub(crate) const fn is_no_chaperone_for_escaped_quote_in_triple_quoted_docstring
 ) -> bool {
     context.is_preview()
 }
+
+/// Returns `true` if the [`blank_line_before_decorated_class_in_stub`](https://github.com/astral-sh/ruff/issues/18865) preview style is enabled.
+pub(crate) const fn is_blank_line_before_decorated_class_in_stub_enabled(
+    context: &PyFormatContext,
+) -> bool {
+    context.is_preview()
+}

crates/ruff_python_formatter/src/statement/suite.rs

@@ -13,6 +13,7 @@ use crate::comments::{
 use crate::context::{NodeLevel, TopLevelStatementPosition, WithIndentLevel, WithNodeLevel};
 use crate::other::string_literal::StringLiteralKind;
 use crate::prelude::*;
+use crate::preview::is_blank_line_before_decorated_class_in_stub_enabled;
 use crate::statement::stmt_expr::FormatStmtExpr;
 use crate::verbatim::{
     suppressed_node, write_suppressed_statements_starting_with_leading_comment,
@@ -700,7 +701,12 @@ fn stub_suite_can_omit_empty_line(preceding: &Stmt, following: &Stmt, f: &PyForm
     //
     // class LockType2: ...
     // ```
-    let class_decorator_instead_of_empty_line = preceding.is_function_def_stmt()
+    //
+    // However, this behavior is incorrect and should not be replicated in preview mode.
+    // See: https://github.com/astral-sh/ruff/issues/18865
+    let class_decorator_instead_of_empty_line =
+        !is_blank_line_before_decorated_class_in_stub_enabled(f.context())
+            && preceding.is_function_def_stmt()
             && following
                 .as_class_def_stmt()
                 .is_some_and(|class| !class.decorator_list.is_empty());

crates/ruff_python_formatter/tests/snapshots/format@stub_files__decorated_class_after_function.pyi.snap

@@ -0,0 +1,46 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/stub_files/decorated_class_after_function.pyi
+---
+## Input
+```python
+# Issue #18865: Decorated classes below functions should be separated with blank lines
+def hello(): ...
+@lambda _, /: _
+class A: ...
+
+def world(): ...
+
+@final
+class B: ...
+```
+
+## Output
+```python
+# Issue #18865: Decorated classes below functions should be separated with blank lines
+def hello(): ...
+@lambda _, /: _
+class A: ...
+
+def world(): ...
+@final
+class B: ...
+```
+
+
+## Preview changes
+```diff
+--- Stable
++++ Preview
+@@ -1,8 +1,10 @@
+ # Issue #18865: Decorated classes below functions should be separated with blank lines
+ def hello(): ...
++
+ @lambda _, /: _
+ class A: ...
+ 
+ def world(): ...
++
+ @final
+ class B: ...
+```

crates/ruff_python_formatter/tests/snapshots/format@stub_files__top_level.pyi.snap

@@ -1,7 +1,6 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
 input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/stub_files/top_level.pyi
-snapshot_kind: text
 ---
 ## Input
 ```python
@@ -43,3 +42,18 @@ class LockType3: ...
 @final
 class LockType4: ...
 ```
+
+
+## Preview changes
+```diff
+--- Stable
++++ Preview
+@@ -4,6 +4,7 @@
+ def count2(): ...
+ @final
+ def count3(): ...
++
+ @final
+ class LockType1: ...
+ 
+```