fix: prevent -- signature delimiter rendering as \ in bubble (#14134)

# Pull Request Template

## Description

Fixes
https://linear.app/chatwoot/issue/CW-6903/signature-delimiter-renders-as-h2-when-using-enter-line-before

**1**. Fixes an issue where the signature delimiter `--` gets parsed as
an H2 when using **Enter** (new paragraph) before or after it, causing
it to render as a bold `\` in the message bubble.

* Ensures `--` renders as plain text
* Aligns renderer with parser behavior (both disable `lheading`)
* Prevents stray `\` from appearing as heading text

**2**. Also fixes a related editor issue where toggling signature
**off** leaves behind a stray `\` or `-- \`.

* Strips blank paragraph markers (`\`) and dangling hard breaks
(`\<newline>`) from ProseMirror serializer
* Applied in both `appendSignature` and `removeSignature`
* Replaces `trimEnd()` with shared helpers (`trimTrailingBlanks` /
`stripTrailingBlankMarkers`)



## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

#### Screenshots

**Before**
<img width="194" height="204" alt="image"
src="https://github.com/user-attachments/assets/b286ab50-7f89-4910-a552-1568902b93b3"
/>

**After**
<img width="194" height="220" alt="image"
src="https://github.com/user-attachments/assets/658cd543-bce2-46e2-a319-35e5374f1aef"
/>

**Editor**

https://linear.app/chatwoot/issue/CW-6903/signature-delimiter-renders-as-in-h2-when-using-enter-line#comment-5814b882

### Steps

#### Editor

1. Enable agent signature
2. Add and remove new lines around the signature using Enter/shift enter
3. Toggle signature off
4. Notice stray `\` or `-- \` remains

#### Bubble

1. Enable agent signature
2. Send a message using Enter between lines
3. Verify `--` renders correctly (no H2, no bold `\`)

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/dashboard/helper/editorHelper.js

@@ -123,6 +123,13 @@ export function cleanSignature(signature) {
   }
 }
 
+// Strip `\<newline>` hardbreak markers trailing `--` after a signature slice
+const stripDelimiterHardbreaks = body =>
+  body.replace(/(--)\s*(?:\\\s*)+$/, '$1');
+
+// Strip standalone blank-paragraph markers (`\` on their own lines).
+const stripTrailingBlankLine = body => body.replace(/\n(?:\s*\\\n)+$/, '');
+
 /**
  * Adds the signature delimiter to the beginning of the signature.
  *
@@ -227,13 +234,19 @@ export function removeSignature(body, signature, channelType) {
   // trimming will ensure any spaces or new lines before the signature are removed
   // This means we will have the delimiter at the end
   if (signatureIndex > -1) {
-    newBody = newBody.substring(0, signatureIndex).trimEnd();
+    newBody = stripDelimiterHardbreaks(
+      newBody.substring(0, signatureIndex)
+    ).trimEnd();
   }
 
   // Remove delimiter if it's at the end
   if (newBody.endsWith(SIGNATURE_DELIMITER)) {
     // if the delimiter is at the end, remove it
     newBody = newBody.slice(0, -SIGNATURE_DELIMITER.length);
+    // strip any trailing blank-line markers
+    if (signatureIndex > -1) {
+      newBody = stripTrailingBlankLine(newBody);
+    }
   }
 
   return newBody;

app/javascript/dashboard/helper/specs/editorHelper.spec.js

@@ -336,6 +336,38 @@ describe('removeSignature', () => {
       'This is a test\n\n'
     );
   });
+  it('strips blank-paragraph marker before the delimiter', () => {
+    expect(removeSignature('hey\n\n\\\n--\n\nHello there', 'Hello there')).toBe(
+      'hey'
+    );
+  });
+  it('strips multiple consecutive blank-paragraph markers before the delimiter', () => {
+    expect(
+      removeSignature('wewe\n\n\\\n\\\n\\\n--\n\nHello there', 'Hello there')
+    ).toBe('wewe');
+  });
+  it('strips dangling hardbreak when signature shared a paragraph with "--"', () => {
+    expect(removeSignature('hey\n\n--\\\nHello there', 'Hello there')).toBe(
+      'hey\n\n'
+    );
+  });
+  it('preserves trailing backslash in user text when appending', () => {
+    expect(appendSignature('The path is C:\\', 'Best\nAgent')).toContain(
+      'C:\\'
+    );
+    expect(appendSignature('C:\\\n', 'Best\nAgent')).toContain('C:\\');
+    expect(appendSignature('C:\\\n\n', 'Best\nAgent')).toContain('C:\\');
+  });
+  it('preserves trailing backslash in user text when removing', () => {
+    expect(removeSignature('C:\\\n--\n\nBest\nAgent', 'Best\nAgent')).toContain(
+      'C:\\'
+    );
+    expect(removeSignature('C:\\\n--', 'no matching sig')).toContain('C:\\');
+    expect(removeSignature('C:\\\nBest\\\nAgent', 'Best\nAgent')).toContain(
+      'C:\\'
+    );
+    expect(removeSignature('notes\n\\\n--', 'no matching sig')).toContain('\\');
+  });
 });
 
 describe('removeSignature with stripped signature', () => {

app/javascript/shared/helpers/MessageFormatter.js

@@ -42,6 +42,7 @@ const createMarkdownInstance = (linkify = true) => {
     quotes: '\u201c\u201d\u2018\u2019',
     maxNesting: 20,
   })
+    .disable(['lheading'])
     .use(mentionPlugin)
     .use(imgResizeManager)
     .use(mila, {

app/javascript/shared/helpers/specs/MessageFormatter.spec.js

@@ -33,6 +33,13 @@ describe('#MessageFormatter', () => {
 <h2>tool</h2>`
       );
     });
+
+    it('should not render a setext heading when text is followed by "--"', () => {
+      const message = 'hy\n\n\\\n\\-\\-\n\nHello there';
+      const result = new MessageFormatter(message).formattedMessage;
+      expect(result).not.toMatch('<h2>');
+      expect(result).not.toMatch('<h1>');
+    });
   });
 
   describe('content with image and has "cw_image_height" query at the end of URL', () => {