assistant: Normalize line endings for prompts loaded from templates (#16808)

Closes #16804

Similar to #15708, when reading prompts from a template, both Windows
and Linux might end up with `CRLF (\r\n)` line endings, which can result
in a panic.

Release Notes:

- N/A

crates/text/src/text.rs

@@ -3041,4 +3041,12 @@ impl LineEnding {
             text
         }
     }
+
+    pub fn normalize_cow(text: Cow<str>) -> Cow<str> {
+        if let Cow::Owned(replaced) = LINE_SEPARATORS_REGEX.replace_all(&text, "\n") {
+            replaced.into()
+        } else {
+            text
+        }
+    }
 }