[red-knot] Do not panic on f-string format spec expressions (#14436)

## Summary

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

## Test Plan

```
cargo nextest run -p red_knot_workspace -- --ignored linter_af linter_gz
```

crates/red_knot_python_semantic/src/types/infer.rs

@@ -2285,6 +2285,12 @@ impl<'db> TypeInferenceBuilder<'db> {
                                 } = expression;
                                 let ty = self.infer_expression(expression);
 
+                                if let Some(ref format_spec) = format_spec {
+                                    for element in format_spec.elements.expressions() {
+                                        self.infer_expression(&element.expression);
+                                    }
+                                }
+
                                 // TODO: handle format specifiers by calling a method
                                 // (`Type::format`?) that handles the `__format__` method.
                                 // Conversion flags should be handled before calling `__format__`.