fix(comments): handle adjacent run boundaries

Use half-open command ranges so a zero-length attachment at the shared boundary of adjacent command runs is rendered exactly once. Add a regression spec for the boundary case.

spec/invidious/videos/description_spec.cr

@@ -147,4 +147,43 @@ Spectator.describe "parse_description" do
       "cd</a>"
     )
   end
+
+  it "assigns boundary attachments to only the following command run" do
+    description = JSON.parse(<<-JSON)
+      {
+        "content": "abcd",
+        "commandRuns": [
+          {
+            "startIndex": 0,
+            "length": 2,
+            "onTap": {"innertubeCommand": {"watchEndpoint": {"videoId": "first-video"}}}
+          },
+          {
+            "startIndex": 2,
+            "length": 2,
+            "onTap": {"innertubeCommand": {"watchEndpoint": {"videoId": "second-video"}}}
+          }
+        ],
+        "attachmentRuns": [{
+          "startIndex": 2,
+          "length": 0,
+          "element": {
+            "type": {
+              "imageType": {
+                "image": {
+                  "sources": [{"url": "https://lh3.googleusercontent.com/emoji-boundary=s16", "width": 16, "height": 16}]
+                }
+              }
+            }
+          },
+          "properties": {"accessibilityProperties": {"label": "emoji-boundary"}}
+        }]
+      }
+    JSON
+
+    expect(parse_description(description, "video-id")).to eq(
+      %(<a href="/watch?v=first-video">ab</a>) +
+      %(<a href="/watch?v=second-video"><img alt="emoji-boundary" src="/ggpht/emoji-boundary=s16" title="emoji-boundary" width="16" height="16" class="channel-emoji" />cd</a>)
+    )
+  end
 end

src/invidious/videos/description.cr

@@ -99,7 +99,9 @@ private def attachment_run_inside_command?(attachment, command) : Bool
   command_start = command["startIndex"].as_i
   command_end = command_start + command["length"].as_i
 
-  attachment_start >= command_start && attachment_end <= command_end
+  attachment_start >= command_start &&
+    attachment_start < command_end &&
+    attachment_end <= command_end
 end
 
 private def parse_command_run(command, nested_attachments, iter, video_id : String) : String