email_notification: Fix bad rendering of math formulas.

Earlier, for the emails having latex math like
"$$d^* = +\infty$$", the bad rendering led to the math
being included multiple times in the email body.

This was due to displaying KaTeX HTML without the CSS.

This commit fixes the incorrect behavior by replacing
the KaTeX with the raw LaTex source.

Fixes part of #25289.

zerver/lib/email_notifications.py

@@ -248,6 +248,20 @@ def build_message_list(
         relative_to_full_url(fragment, user.realm.uri)
         fix_emojis(fragment, user.emojiset)
         fix_spoilers_in_html(fragment, user.default_language)
+
+        # Selecting the <span> elements with class 'katex'
+        katex_spans = fragment.xpath("//span[@class='katex']")
+
+        # Iterate through 'katex_spans' and replace with a new <span> having LaTeX text.
+        for katex_span in katex_spans:
+            latex_text = katex_span.xpath(".//annotation[@encoding='application/x-tex']")[0].text
+            # We store 'tail' to insert them back as the replace operation removes it.
+            tail = katex_span.tail
+            latex_span = lxml.html.Element("span")
+            latex_span.text = f"$${latex_text}$$"
+            katex_span.getparent().replace(katex_span, latex_span)
+            latex_span.tail = tail
+
         html = lxml.html.tostring(fragment, encoding="unicode")
         if sender:
             plain, html = prepend_sender_to_message(plain, html, sender)

zerver/tests/test_message_notification_emails.py

@@ -1593,6 +1593,20 @@ class TestMessageNotificationEmails(ZulipTestCase):
         )
         self.assertEqual(actual_output, expected_output)
 
+    def test_latex_math_formulas_in_email(self) -> None:
+        msg_id = self.send_stream_message(
+            self.example_user("iago"), "Denmark", "Equation: $$d^* = +\\infty$$ is correct."
+        )
+        verify_body_include = ["Equation: <span>$$d^* = +\\infty$$</span> is correct"]
+        email_subject = "#Denmark > test"
+        self._test_cases(
+            msg_id,
+            verify_body_include,
+            email_subject,
+            verify_html_body=True,
+            trigger=NotificationTriggers.STREAM_EMAIL,
+        )
+
     def test_empty_backticks_in_missed_message(self) -> None:
         msg_id = self.send_personal_message(
             self.example_user("othello"),