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/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', () => {