chore: address Rust 1.78 clippy lints (#4545)

Co-authored-by: hyperswitch-bot[bot] <148525504+hyperswitch-bot[bot]@users.noreply.github.com>

crates/router_derive/src/macros/api_error/helpers.rs

@@ -260,9 +260,9 @@ pub(super) fn get_unused_fields(
     ignore: &std::collections::HashSet<String>,
 ) -> Vec<Field> {
     let fields = match fields {
-        syn::Fields::Unit => Vec::new(),
-        syn::Fields::Unnamed(_) => Vec::new(),
-        syn::Fields::Named(fields) => fields.named.iter().cloned().collect(),
+        Fields::Unit => Vec::new(),
+        Fields::Unnamed(_) => Vec::new(),
+        Fields::Named(fields) => fields.named.iter().cloned().collect(),
     };
     fields
         .iter()

crates/router_derive/src/macros/helpers.rs

@@ -58,7 +58,7 @@ pub(super) fn get_struct_fields(
         Ok(named.to_owned())
     } else {
         Err(syn::Error::new(
-            proc_macro2::Span::call_site(),
+            Span::call_site(),
             "This macro cannot be used on structs with no fields",
         ))
     }

crates/router_derive/src/macros/try_get_enum.rs

@@ -68,7 +68,7 @@ pub fn try_get_enum_variant(
 
         let try_into_fn = syn::Ident::new(
             &format!("try_into_{}", variant_name.to_string().to_lowercase()),
-            proc_macro2::Span::call_site(),
+            Span::call_site(),
         );
 
         Ok(quote::quote! {