Ensure PR description is not too long

With the addition of structured metadata to PR description the content was growing too large when multiple updates got grouped together.
Now the metadata gets only appended, if the length is still below 65536 characters.

Fixes: #3476

Author: mzuehlke

modules/core/src/main/scala/org/scalasteward/core/forge/data/NewPullRequestData.scala

@@ -96,35 +96,45 @@ object NewPullRequestData {
       _ => ""
     )
 
-    s"""|$updatesText
-        |
-        |## Usage
-        |✅ **Please merge!**
-        |
-        |I'll automatically update this PR to resolve conflicts as long as you don't change it yourself.
-        |
-        |${skipVersionMessage}If you have any feedback, just mention me in the comments below.
-        |
-        |Configure Scala Steward for your repository with a [`${RepoConfigAlg.repoConfigBasename}`](${org.scalasteward.core.BuildInfo.gitHubUrl}/blob/${org.scalasteward.core.BuildInfo.gitHeadCommit}/docs/repo-specific-configuration.md) file.
-        |
-        |_Have a fantastic day writing Scala!_
-        |
-        |${details.map(_.toHtml).mkString("\n")}
-        |
-        |<sup>
-        |${labels.mkString("labels: ", ", ", "")}
-        |</sup>
-        |
-        |<!-- scala-steward = ${metadataJson(update, labels)} -->""".stripMargin.trim
-  }
+    val plainBody =
+      s"""|$updatesText
+          |
+          |## Usage
+          |✅ **Please merge!**
+          |
+          |I'll automatically update this PR to resolve conflicts as long as you don't change it yourself.
+          |
+          |${skipVersionMessage}If you have any feedback, just mention me in the comments below.
+          |
+          |Configure Scala Steward for your repository with a [`${RepoConfigAlg.repoConfigBasename}`](${org.scalasteward.core.BuildInfo.gitHubUrl}/blob/${org.scalasteward.core.BuildInfo.gitHeadCommit}/docs/repo-specific-configuration.md) file.
+          |
+          |_Have a fantastic day writing Scala!_
+          |
+          |${details.map(_.toHtml).mkString("\n")}
+          |
+          |<sup>
+          |${labels.mkString("labels: ", ", ", "")}
+          |</sup>
+          |""".stripMargin.trim
 
-  def metadataJson(update: Update, labels: List[String]): String =
-    Json
-      .obj(
-        "Update" -> update.asJson,
-        "Labels" -> Json.fromValues(labels.map(_.asJson))
-      )
-      .toString
+    val metadataJson =
+      Json
+        .obj(
+          "Update" -> update.asJson,
+          "Labels" -> Json.fromValues(labels.map(_.asJson))
+        )
+        .toString
+
+    val bodyWithMetadata =
+      s"""$plainBody
+         |
+         |<!-- scala-steward = $metadataJson -->""".stripMargin
+
+    // Github limits PR descriptions to 65536 unicode characters
+    if (bodyWithMetadata.length < 65536)
+      bodyWithMetadata
+    else plainBody
+  }
 
   def renderUpdateInfoUrls(updateInfoUrls: List[UpdateInfoUrl]): Option[String] =
     Option.when(updateInfoUrls.nonEmpty) {