FIX: correctly extract body and/or reply from exchange emails (#30512)

When receiving emails sent with Exchange, we look for some markers to identify the body of the mail and the reply (aka. previous email).

For some reasons, those markers aren't 100% reliable and sometimes, only one of them is present.

The commit 20ba54d53630b57c25fa3f325b0f219581314936 introduced the bug because the `HTML_EXTRACTERS` regex for exchange looks for either `messageBodySection` or `messageReplySection` but we were only using the `reply` section. So if an email had only the `body` section, it would not be correctly extracted.

This commit handle the cases where either one of them is missing and use the other one as the actual "reply". When both are present, it correctly elides the "reply" section.

spec/lib/email/receiver_spec.rb

@@ -415,9 +415,43 @@ RSpec.describe Email::Receiver do
     it "automatically elides gmail quotes" do
       SiteSetting.always_show_trimmed_content = true
       expect { process(:gmail_html_reply) }.to change { topic.posts.count }
-      expect(topic.posts.last.raw).to eq(
-        "This is a **GMAIL** reply ;)\n\n<details class='elided'>\n<summary title='Show trimmed content'>&#183;&#183;&#183;</summary>\n\nThis is the *elided* part!\n\n</details>",
-      )
+      expect(topic.posts.last.raw).to eq <<~MD.strip
+        This is a **GMAIL** reply ;)
+        
+        <details class='elided'>
+        <summary title='Show trimmed content'>&#183;&#183;&#183;</summary>
+        
+        This is the *elided* part!
+        
+        </details>
+      MD
+    end
+
+    it "correctly extracts body from exchange emails" do
+      SiteSetting.always_show_trimmed_content = true
+      expect { process(:exchange_html_body) }.to change { topic.posts.count }
+      expect(topic.posts.last.raw).to eq("This is the **body** of the email.")
+    end
+
+    it "correctly extracts reply from exchange emails" do
+      SiteSetting.always_show_trimmed_content = true
+      expect { process(:exchange_html_reply) }.to change { topic.posts.count }
+      expect(topic.posts.last.raw).to eq("This is the **body !!** of the email.")
+    end
+
+    it "correctly extracts body & reply from exchange emails" do
+      SiteSetting.always_show_trimmed_content = true
+      expect { process(:exchange_html_body_and_reply) }.to change { topic.posts.count }
+      expect(topic.posts.last.raw).to eq <<~MD.strip
+        This is the **body** of the email.
+
+        <details class='elided'>
+        <summary title='Show trimmed content'>&#183;&#183;&#183;</summary>
+
+        This is the *reply*!
+
+        </details>
+      MD
     end
 
     it "doesn't process email with same message-id more than once" do