Avoid parenthesizing comprehension element (#6198)

## Summary

This PR adds a new precedence level for the comprehension element. This fixes
the generator to not add parentheses around the comprehension element every
time.

The new precedence level is `COMPREHENSION_ELEMENT` and it should occur after
the `NAMED_EXPR` precedence level because named expressions are always parenthesized.

This matches the behavior of Python `ast.unparse` and tested with the
following snippet:

```python
import ast

code = ""
ast.unparse(ast.parse(code))
```

## Test Plan

Add a bunch of test cases for all the valid nodes at that position.

fixes: #5777

crates/ruff_python_codegen/src/generator.rs

@@ -34,6 +34,7 @@ mod precedence {
     pub(crate) const COMMA: u8 = 21;
     pub(crate) const NAMED_EXPR: u8 = 23;
     pub(crate) const ASSERT: u8 = 23;
+    pub(crate) const COMPREHENSION_ELEMENT: u8 = 27;
     pub(crate) const LAMBDA: u8 = 27;
     pub(crate) const IF_EXP: u8 = 27;
     pub(crate) const COMPREHENSION: u8 = 29;
@@ -1052,7 +1053,7 @@ impl<'a> Generator<'a> {
                 range: _range,
             }) => {
                 self.p("[");
-                self.unparse_expr(elt, precedence::MAX);
+                self.unparse_expr(elt, precedence::COMPREHENSION_ELEMENT);
                 self.unparse_comp(generators);
                 self.p("]");
             }
@@ -1062,7 +1063,7 @@ impl<'a> Generator<'a> {
                 range: _range,
             }) => {
                 self.p("{");
-                self.unparse_expr(elt, precedence::MAX);
+                self.unparse_expr(elt, precedence::COMPREHENSION_ELEMENT);
                 self.unparse_comp(generators);
                 self.p("}");
             }
@@ -1073,9 +1074,9 @@ impl<'a> Generator<'a> {
                 range: _range,
             }) => {
                 self.p("{");
-                self.unparse_expr(key, precedence::MAX);
+                self.unparse_expr(key, precedence::COMPREHENSION_ELEMENT);
                 self.p(": ");
-                self.unparse_expr(value, precedence::MAX);
+                self.unparse_expr(value, precedence::COMPREHENSION_ELEMENT);
                 self.unparse_comp(generators);
                 self.p("}");
             }
@@ -1085,7 +1086,7 @@ impl<'a> Generator<'a> {
                 range: _range,
             }) => {
                 self.p("(");
-                self.unparse_expr(elt, precedence::COMMA);
+                self.unparse_expr(elt, precedence::COMPREHENSION_ELEMENT);
                 self.unparse_comp(generators);
                 self.p(")");
             }
@@ -1570,6 +1571,8 @@ mod tests {
         assert_round_trip!("foo(1)");
         assert_round_trip!("foo(1, 2)");
         assert_round_trip!("foo(x for x in y)");
+        assert_round_trip!("foo([x for x in y])");
+        assert_round_trip!("foo([(x := 2) for x in y])");
         assert_round_trip!("x = yield 1");
         assert_round_trip!("return (yield 1)");
         assert_round_trip!("lambda: (1, 2, 3)");
@@ -1622,8 +1625,8 @@ mod tests {
             r#"def f() -> (int, str):
     pass"#
         );
-        assert_round_trip!("[(await x) async for x in y]");
-        assert_round_trip!("[(await i) for i in b if await c]");
+        assert_round_trip!("[await x async for x in y]");
+        assert_round_trip!("[await i for i in b if await c]");
         assert_round_trip!("(await x async for x in y)");
         assert_round_trip!(
             r#"async def read_data(db):
@@ -1719,6 +1722,18 @@ class Foo:
         pass"#
         );
 
+        assert_round_trip!(r#"[lambda n: n for n in range(10)]"#);
+        assert_round_trip!(r#"[n[0:2] for n in range(10)]"#);
+        assert_round_trip!(r#"[n[0] for n in range(10)]"#);
+        assert_round_trip!(r#"[(n, n * 2) for n in range(10)]"#);
+        assert_round_trip!(r#"[1 if n % 2 == 0 else 0 for n in range(10)]"#);
+        assert_round_trip!(r#"[n % 2 == 0 or 0 for n in range(10)]"#);
+        assert_round_trip!(r#"[(n := 2) for n in range(10)]"#);
+        assert_round_trip!(r#"((n := 2) for n in range(10))"#);
+        assert_round_trip!(r#"[n * 2 for n in range(10)]"#);
+        assert_round_trip!(r#"{n * 2 for n in range(10)}"#);
+        assert_round_trip!(r#"{i: n * 2 for i, n in enumerate(range(10))}"#);
+
         // Type aliases
         assert_round_trip!(r#"type Foo = int | str"#);
         assert_round_trip!(r#"type Foo[T] = list[T]"#);