FIX: proper details / summary excerpt (#30229)

It doesn't make much sense to have the content of a `<details>` in an excerpt so I replaced them with "▶ summary" instead.

That way, they can't be (ab)used in user cards for example.

Reference - https://meta.discourse.org/t/335094

lib/excerpt_parser.rb

@@ -24,8 +24,6 @@ class ExcerptParser < Nokogiri::XML::SAX::Document
     @start_excerpt = false
     @start_hashtag_icon = false
     @in_details_depth = 0
-    @summary_contents = +""
-    @detail_contents = +""
   end
 
   def self.get_excerpt(html, length, options)
@@ -127,12 +125,11 @@ class ExcerptParser < Nokogiri::XML::SAX::Document
         include_tag(name, attributes)
       end
     when "details"
-      @detail_contents = +"" if @in_details_depth == 0
       @in_details_depth += 1
     when "summary"
       if @in_details_depth == 1 && !@in_summary
-        @summary_contents = +""
         @in_summary = true
+        characters("▶ ", truncate: false, count_it: false, encode: false)
       end
     when "svg"
       attributes = Hash[*attributes.flatten]
@@ -162,29 +159,6 @@ class ExcerptParser < Nokogiri::XML::SAX::Document
       @in_quote = false
     when "details"
       @in_details_depth -= 1
-      if @in_details_depth == 0
-        @summary_contents = clean(@summary_contents)
-        @detail_contents = clean(@detail_contents)
-
-        if @current_length + @summary_contents.length >= @length
-          characters(
-            @summary_contents,
-            encode: false,
-            before_string: "<details class='disabled'><summary>",
-            after_string: "</summary></details>",
-          )
-        else
-          characters(
-            @summary_contents,
-            truncate: false,
-            encode: false,
-            before_string: "<details><summary>",
-            after_string: "</summary>",
-          )
-
-          characters(@detail_contents, encode: false, after_string: "</details>")
-        end
-      end
     when "summary"
       @in_summary = false if @in_details_depth == 1
     when "div", "span"
@@ -210,18 +184,10 @@ class ExcerptParser < Nokogiri::XML::SAX::Document
     before_string: nil,
     after_string: nil
   )
-    return if @in_quote
+    return if @in_quote || @in_details_depth > 1 || (@in_details_depth == 1 && !@in_summary)
 
     # we call length on this so might as well ensure we have a string
     string = string.to_s
-    if @in_details_depth > 0
-      if @in_summary
-        @summary_contents << string
-      else
-        @detail_contents << string
-      end
-      return
-    end
 
     @excerpt << before_string if before_string