add dedicated small fixtures for fluent formatting

crates/ruff_python_formatter/resources/test/fixtures/ruff/fluent.options.json

@@ -0,0 +1 @@
+[{"line_width":8}]

crates/ruff_python_formatter/resources/test/fixtures/ruff/fluent.py

@@ -0,0 +1,35 @@
+# Fixtures for fluent formatting of call chains
+# Note that `fluent.options.json` sets line width to 8
+
+
+x = a.b()
+
+x = a.b().c()
+
+x = a.b().c().d
+
+x = a.b.c.d().e()
+
+x = a.b.c().d.e().f.g()
+
+# Consecutive calls/subscripts are grouped together
+# for the purposes of fluent formatting (though, as 2025.12.15,
+# there may be a break inside of one of these
+# calls/subscripts, but that is unrelated to the fluent format.)
+
+x = a()[0]().b().c()
+
+x = a.b()[0].c.d()[1]().e
+
+# Parentheses affect both where the root of the call
+# chain is and how many calls we require before applying
+# fluent formatting (just 1, in the presence of a parenthesized
+# root, as of 2025.12.15.)
+
+x = (a).b()
+
+x = (a()).b()
+
+x = (a.b()).d.e()
+
+x = (a.b().d).e()

crates/ruff_python_formatter/tests/snapshots/format@fluent.py.snap

@@ -0,0 +1,163 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/fluent.py
+---
+## Input
+```python
+# Fixtures for fluent formatting of call chains
+# Note that `fluent.options.json` sets line width to 8
+
+
+x = a.b()
+
+x = a.b().c()
+
+x = a.b().c().d
+
+x = a.b.c.d().e()
+
+x = a.b.c().d.e().f.g()
+
+# Consecutive calls/subscripts are grouped together
+# for the purposes of fluent formatting (though, as 2025.12.15,
+# there may be a break inside of one of these
+# calls/subscripts, but that is unrelated to the fluent format.)
+
+x = a()[0]().b().c()
+
+x = a.b()[0].c.d()[1]().e
+
+# Parentheses affect both where the root of the call
+# chain is and how many calls we require before applying
+# fluent formatting (just 1, in the presence of a parenthesized
+# root, as of 2025.12.15.)
+
+x = (a).b()
+
+x = (a()).b()
+
+x = (a.b()).d.e()
+
+x = (a.b().d).e()
+```
+
+## Outputs
+### Output 1
+```
+indent-style               = space
+line-width                 = 8
+indent-width               = 4
+quote-style                = Double
+line-ending                = LineFeed
+magic-trailing-comma       = Respect
+docstring-code             = Disabled
+docstring-code-line-width  = "dynamic"
+preview                    = Disabled
+target_version             = 3.10
+source_type                = Python
+```
+
+```python
+# Fixtures for fluent formatting of call chains
+# Note that `fluent.options.json` sets line width to 8
+
+
+x = a.b()
+
+x = a.b().c()
+
+x = (
+    a.b()
+    .c()
+    .d
+)
+
+x = a.b.c.d().e()
+
+x = (
+    a.b.c()
+    .d.e()
+    .f.g()
+)
+
+# Consecutive calls/subscripts are grouped together
+# for the purposes of fluent formatting (though, as 2025.12.15,
+# there may be a break inside of one of these
+# calls/subscripts, but that is unrelated to the fluent format.)
+
+x = (
+    a()[
+        0
+    ]()
+    .b()
+    .c()
+)
+
+x = (
+    a.b()[
+        0
+    ]
+    .c.d()[
+        1
+    ]()
+    .e
+)
+
+# Parentheses affect both where the root of the call
+# chain is and how many calls we require before applying
+# fluent formatting (just 1, in the presence of a parenthesized
+# root, as of 2025.12.15.)
+
+x = (
+    a
+).b()
+
+x = (
+    a()
+).b()
+
+x = (
+    a.b()
+).d.e()
+
+x = (
+    a.b().d
+).e()
+```
+
+
+#### Preview changes
+```diff
+--- Stable
++++ Preview
+@@ -7,7 +7,8 @@
+ x = a.b().c()
+ 
+ x = (
+-    a.b()
++    a
++    .b()
+     .c()
+     .d
+ )
+@@ -15,7 +16,8 @@
+ x = a.b.c.d().e()
+ 
+ x = (
+-    a.b.c()
++    a.b
++    .c()
+     .d.e()
+     .f.g()
+ )
+@@ -34,7 +36,8 @@
+ )
+ 
+ x = (
+-    a.b()[
++    a
++    .b()[
+         0
+     ]
+     .c.d()[
+```