fix(java): address review feedback for system message sections

Co-authored-by: edburns <75821+edburns@users.noreply.github.com>

Author: Copilot

java/src/main/java/com/github/copilot/rpc/SystemMessageSections.java

@@ -25,7 +25,7 @@
  * @see SectionOverride
  * @since 1.0.2
  */
-public sealed class SystemMessageSections permits SystemPromptSections {
+public abstract sealed class SystemMessageSections permits SystemPromptSections {
 
     /** Agent identity preamble and mode statement. */
     public static final String IDENTITY = "identity";

java/src/main/java/com/github/copilot/rpc/SystemPromptSections.java

@@ -13,7 +13,7 @@
  * @deprecated Use {@link SystemMessageSections} — this class will be removed in
  *             a future major version.
  * @see SystemMessageSections
- * @since 1.2.0
+ * @since 1.0.2
  */
 @Deprecated(since = "1.0.2", forRemoval = true)
 public final class SystemPromptSections extends SystemMessageSections {