FIX: Remove newlines from img alt & title in HTML to markdown parser (#25473)

We were having a minor issue with emails with embedded images
that had newlines in the alt string; for example:

```
<p class="MsoNormal"><span style="font-size:11.0pt"><img width="898"
height="498" style="width:9.3541in;height:5.1875in" id="Picture_x0020_5"
src="cid:image003.png@01DA4EBA.0400B610" alt="A screenshot of a computer
program

Description automatically generated"></span><span
style="font-size:11.0pt"><o:p></o:p></span></p>
```

Once this was parsed and converted to markdown (or directly to HTML
in some cases), this caused an issue in the composer and the post
UI, where the markdown parser didn't know how to deal with this,
making the HTML show directly instead of showing an image.

The easiest way to deal with this is to just strip \n from image
alt and title attrs in the HTMLToMarkdown class.

lib/html_to_markdown.rb

@@ -21,6 +21,10 @@ class HtmlToMarkdown
 
   private
 
+  def strip_newlines(string)
+    string.gsub(/\n/, " ")&.squeeze(" ")
+  end
+
   def remove_not_allowed!(doc)
     allowed = Set.new
 
@@ -167,16 +171,18 @@ class HtmlToMarkdown
   def visit_img(node)
     return if node["src"].blank?
 
+    node["alt"] = strip_newlines(node["alt"]) if node["alt"].present?
+    node["title"] = strip_newlines(node["title"]) if node["title"].present?
+
     if @opts[:keep_img_tags]
       node.to_html
     elsif @opts[:keep_cid_imgs] && node["src"].start_with?("cid:")
       node.to_html
     elsif node["src"].start_with?(*ALLOWED_IMG_SRCS)
-      title = node["alt"].presence || node["title"].presence
       width = node["width"].to_i
       height = node["height"].to_i
       dimensions = "|#{width}x#{height}" if width > 0 && height > 0
-      "![#{title}#{dimensions}](#{node["src"]})"
+      "![#{node["alt"] || node["title"]}#{dimensions}](#{node["src"]})"
     end
   end
 

spec/lib/html_to_markdown_spec.rb

@@ -150,6 +150,14 @@ RSpec.describe HtmlToMarkdown do
     expect(html_to_markdown(HTML_WITH_IMG, keep_img_tags: true)).to eq(HTML_WITH_IMG)
   end
 
+  it "removes newlines from img alt text" do
+    html_with_alt_newlines =
+      %Q{<img src="https://www.discourse.org/logo.svg" alt="Discourse\n\nLogo">}
+    expect(html_to_markdown(html_with_alt_newlines)).to eq(
+      "![Discourse Logo](https://www.discourse.org/logo.svg)",
+    )
+  end
+
   it "removes empty & invalid <img>" do
     expect(html_to_markdown("<img>")).to eq("")
     expect(html_to_markdown(%Q{<img src="">})).to eq("")
@@ -160,6 +168,13 @@ RSpec.describe HtmlToMarkdown do
     expect(html_to_markdown(HTML_WITH_CID_IMG, keep_cid_imgs: true)).to eq(HTML_WITH_CID_IMG)
   end
 
+  it "removes newlines from img alt text with cid images" do
+    html_with_cid_alt_newlines = %Q{<img src="cid:ii_1525434659ddb4cb" title="Discourse\n\nLogo">}
+    expect(html_to_markdown(html_with_cid_alt_newlines, keep_cid_imgs: true)).to eq(
+      %Q{<img src="cid:ii_1525434659ddb4cb" title="Discourse Logo">},
+    )
+  end
+
   it "skips hidden <img>" do
     expect(html_to_markdown(%Q{<img src="https://www.discourse.org/logo.svg" width=0>})).to eq("")
     expect(html_to_markdown(%Q{<img src="https://www.discourse.org/logo.svg" height="0">})).to eq(