fix(text/unstable): only strip single trailing newline in dedent

text/unstable_dedent.ts

@@ -16,7 +16,7 @@ const WHITE_SPACE_ONLY_LINE_REGEXP = new RegExp(
  * Removes indentation from multiline strings.
  *
  * - Removes leading newline
- * - Removes trailing whitespace, including newlines
+ * - Removes a single trailing newline (with any preceding whitespace on that line)
  * - Replaces whitespace-only lines with empty lines
  * - Finds the minimum indentation among remaining lines and removes that much indentation from all of them
  *
@@ -44,7 +44,7 @@ export function dedent(input: string): string;
  * Removes indentation from multiline strings.
  *
  * - Removes leading newline
- * - Removes trailing whitespace, including newlines
+ * - Removes a single trailing newline (with any preceding whitespace on that line)
  * - Replaces whitespace-only lines with empty lines
  * - Finds the minimum indentation among remaining lines and removes that much indentation from all of them
  *
@@ -76,7 +76,10 @@ export function dedent(
   // Substitute nonempty placeholder so multiline substitutions do not affect indent width.
   const joinedTemplate = typeof input === "string" ? input : input.join("x");
   const ignoreFirstUnindented = !joinedTemplate.startsWith("\n");
-  const trimmedTemplate = joinedTemplate.replace(/^\n/, "").trimEnd();
+  const trimmedTemplate = joinedTemplate.replace(/^\n/, "").replace(
+    /\n[\t ]*$/,
+    "",
+  );
   const lines = trimmedTemplate.split("\n");
 
   const linesToCheck = lines.slice(
@@ -90,7 +93,7 @@ export function dedent(
   const inputString = typeof input === "string"
     ? input
     : String.raw({ raw: input }, ...values);
-  const trimmedInput = inputString.replace(/^\n/, "").trimEnd();
+  const trimmedInput = inputString.replace(/^\n/, "").replace(/\n[\t ]*$/, "");
 
   // No lines to indent
   if (!indent) return trimmedInput;

text/unstable_dedent_test.ts

@@ -22,6 +22,15 @@ Deno.test("dedent() handles example 2", () => {
   );
 });
 
+// Test case for issue #6831
+Deno.test("dedent() only strips single trailing newline", () => {
+  const result = dedent`
+      a
+
+      `;
+  assertEquals(result, "a\n");
+});
+
 Deno.test("dedent() handles empty lines", () => {
   assertEquals(
     dedent(`