[Fleet] add escapeMultilineString Handlebar helper (elastic#195159)

## Summary

Adding a handlebar helper to escape multiline strings. It has the same
function as `escapeStringHelper`, but does not wrap strings in single
quotes, allowing concatenation of escaped variables in the `hbs`
template such as this example:

```hbs
audit_rules: "{{escape_multiline_string audit_rules}}
  {{escape_multiline_string "
  # Session data audit rules
  -a always,exit -F arch=b64 -S execve,execveat -k exec
  -a always,exit -F arch=b64 -S exit_group
  -a always,exit -F arch=b64 -S setsid"}}"
{{else}}
  {{#if audit_rules}}
audit_rules: {{escape_string audit_rules}}
  {{/if}}
{{/if}}
```

The above would not be possible using only `escape_string` as
`audit_rules` would be wrapped in single quotes.

## Screenshots

The example above illustrates how this option allows the Auditd manager
integration to append Session data audit rules to the `audit_rules`
field when Session data is enabled in the integration
([PR](elastic/integrations#11336)).

<img width="872" alt="image"
src="https://github.com/user-attachments/assets/325d784d-26a4-4dfe-9d0e-54d51c3ed060">

<img width="801" alt="image"
src="https://github.com/user-attachments/assets/ebd521c0-4471-48f9-ba8d-94630ea0efd2">

x-pack/plugins/fleet/server/services/epm/agent/agent.test.ts

@@ -261,6 +261,80 @@ New lines and \\n escaped values.`,
     });
   });
 
+  describe('escape_multiline_string helper', () => {
+    it('should escape new lines', () => {
+      const streamTemplate = `
+      input: log
+      multiline_text: "{{escape_multiline_string multiline_text}}"
+            `;
+
+      const vars = {
+        multiline_text: {
+          type: 'textarea',
+          value: `This is a text with
+New lines and \n escaped values.`,
+        },
+      };
+
+      const output = compileTemplate(vars, streamTemplate);
+      expect(output).toEqual({
+        input: 'log',
+        multiline_text: `This is a text with
+New lines and 
+escaped values.`,
+      });
+    });
+
+    it('should escape single quotes', () => {
+      const streamTemplate = `
+      input: log
+      multiline_text: "{{escape_multiline_string multiline_text}}"
+            `;
+
+      const vars = {
+        multiline_text: {
+          type: 'textarea',
+          value: `This is a multiline text with
+'escaped values.'`,
+        },
+      };
+
+      const output = compileTemplate(vars, streamTemplate);
+      expect(output).toEqual({
+        input: 'log',
+        multiline_text: `This is a multiline text with
+''escaped values.''`,
+      });
+    });
+
+    it('should allow concatenation of multiline strings', () => {
+      const streamTemplate = `
+input: log
+multiline_text: "{{escape_multiline_string multiline_text}}{{escape_multiline_string "
+This is a concatenated text
+with new lines"}}"
+      `;
+
+      const vars = {
+        multiline_text: {
+          type: 'textarea',
+          value: `This is a text with
+New lines and\nescaped values.`,
+        },
+      };
+
+      const output = compileTemplate(vars, streamTemplate);
+      expect(output).toEqual({
+        input: 'log',
+        multiline_text: `This is a text with
+New lines and
+escaped values.
+This is a concatenated text
+with new lines`,
+      });
+    });
+  });
+
   describe('to_json helper', () => {
     const streamTemplate = `
 input: log

x-pack/plugins/fleet/server/services/epm/agent/agent.ts

@@ -131,6 +131,18 @@ function escapeStringHelper(str: string) {
 }
 handlebars.registerHelper('escape_string', escapeStringHelper);
 
+/**
+ * escapeMultilineStringHelper will escape a multiline string by doubling the newlines
+ * and escaping single quotes.
+ * This is useful when the string is multiline and needs to be escaped in a yaml file
+ * without wrapping it in single quotes.
+ */
+function escapeMultilineStringHelper(str: string) {
+  if (!str) return undefined;
+  return str.replace(/\'/g, "''").replace(/\n/g, '\n\n');
+}
+handlebars.registerHelper('escape_multiline_string', escapeMultilineStringHelper);
+
 // toJsonHelper will convert any object to a Json string.
 function toJsonHelper(value: any) {
   if (typeof value === 'string') {