registers: Use saved values for clipboard providers which can't read

This fixes reading from the clipboard when using the termcode provider.
Reading isn't implemented for the termcode provider so `get_contents`
returns `ClipboardError::ReadingNotSupported`. `read_from_clipboard`
needs to recognize this case and use the saved values instead of
emitting an error log and returning nothing.

Follow-up of #10839
Also see #12142

helix-view/src/register.rs

@@ -5,7 +5,7 @@ use arc_swap::access::DynAccess;
 use helix_core::NATIVE_LINE_ENDING;
 
 use crate::{
-    clipboard::{ClipboardProvider, ClipboardType},
+    clipboard::{ClipboardError, ClipboardProvider, ClipboardType},
     Editor,
 };
 
@@ -238,6 +238,10 @@ fn read_from_clipboard<'a>(
                 RegisterValues::new(iter::once(contents.into()))
             }
         }
+        Err(ClipboardError::ReadingNotSupported) => match saved_values {
+            Some(values) => RegisterValues::new(values.iter().map(Cow::from).rev()),
+            None => RegisterValues::new(iter::empty()),
+        },
         Err(err) => {
             log::error!(
                 "Failed to read {} clipboard: {err}",