FIX: need to be able to search replace within lines (#1110)

(this is needed for very simple diffs and HTML)

lib/utils/diff_utils/simple_diff.rb

@@ -19,6 +19,9 @@ module DiscourseAi
           raise ArgumentError, "content cannot be nil" if content.nil?
           raise ArgumentError, "search cannot be nil" if search.nil?
           raise ArgumentError, "replace cannot be nil" if replace.nil?
+          raise ArgumentError, "search cannot be empty" if search.empty?
+
+          return content.gsub(search, replace) if content.include?(search)
 
           lines = content.split("\n")
           search_lines = search.split("\n")

spec/lib/utils/diff_utils/simple_diff_spec.rb

@@ -25,7 +25,7 @@ RSpec.describe DiscourseAi::Utils::DiffUtils::SimpleDiff do
         lin 1
       TEXT
 
-      expect(subject.apply(content, search, replace)).to eq(expected.strip)
+      expect(subject.apply(content, search, replace).strip).to eq(expected.strip)
     end
 
     it "raises error when no match is found" do
@@ -61,10 +61,10 @@ RSpec.describe DiscourseAi::Utils::DiffUtils::SimpleDiff do
     end
 
     it "is forgiving of whitespace differences" do
-      content = "line1\n  line2\nline3"
+      content = "line1\n line2\nline3"
       search = "line2"
       replace = "new_line2"
-      expect(subject.apply(content, search, replace)).to eq("line1\nnew_line2\nline3")
+      expect(subject.apply(content, search, replace).strip).to eq("line1\n new_line2\nline3")
     end
 
     it "is forgiving of small character differences" do
@@ -121,6 +121,13 @@ RSpec.describe DiscourseAi::Utils::DiffUtils::SimpleDiff do
       expect(subject.apply(content, search, replace)).to eq(expected.strip)
     end
 
+    it "handles partial line matches" do
+      content = "abc hello efg\nabc hello efg"
+      search = "hello"
+      replace = "bob"
+      expect(subject.apply(content, search, replace)).to eq("abc bob efg\nabc bob efg")
+    end
+
     it "handles JavaScript blocks in different orders" do
       content = <<~JS
         function first() {