[red-knot] Do not panic on f-string format spec expressions

Previously, we panicked on expressions like `f"{v:{f'0.2f'}}"` because
we did not infer types for expressions nested inside format spec
elements.

crates/red_knot_python_semantic/src/types/infer.rs

@@ -2289,7 +2289,16 @@ impl<'db> TypeInferenceBuilder<'db> {
                                 // (`Type::format`?) that handles the `__format__` method.
                                 // Conversion flags should be handled before calling `__format__`.
                                 // https://docs.python.org/3/library/string.html#format-string-syntax
-                                if !conversion.is_none() || format_spec.is_some() {
+                                if !conversion.is_none() {
+                                    collector.add_expression();
+                                } else if let Some(ref format_spec) = format_spec {
+                                    for element in &format_spec.elements {
+                                        if let ast::FStringElement::Expression(expression) = element
+                                        {
+                                            self.infer_expression(&expression.expression);
+                                        }
+                                    }
+
                                     collector.add_expression();
                                 } else {
                                     if let Type::StringLiteral(literal) = ty.str(self.db) {

crates/red_knot_workspace/tests/check.rs

@@ -283,9 +283,7 @@ const KNOWN_FAILURES: &[(&str, bool, bool)] = &[
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F821_20.py", true, true),
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F821_26.py", true, false),
     // Fails for unknown reasons:
-    ("crates/ruff_linter/resources/test/fixtures/pyflakes/F541.py", true, true),
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F632.py", true, true),
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F811_19.py", true, false),
     ("crates/ruff_linter/resources/test/fixtures/pyupgrade/UP039.py", true, false),
-    ("crates/ruff_python_parser/resources/valid/expressions/f_string.py", true, true),
 ];