fix: Focus API tracker on public API only, eliminate false positives

- Remove internal package scanning (jdk.sandbox.internal.util.json)
- Track only public API classes (jdk.sandbox.java.util.json)
- Eliminate false positive 404 errors for removed internal classes like StableValue
- Reduce tracked classes from 19 to 10 (public API only)
- Improve performance and focus on meaningful API drift detection
- Update tests to verify no internal classes are tracked

Results: 0 missing upstream, 10 public API classes, faster execution

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: simbo1905

json-java21-api-tracker/src/main/java/io/github/simbo1905/tracker/ApiTracker.java

@@ -82,10 +82,9 @@ static Set<Class<?>> discoverLocalJsonClasses() {
         LOGGER.info("Starting class discovery for JSON API packages");
         final var classes = new TreeSet<Class<?>>((a, b) -> a.getName().compareTo(b.getName()));
 
-        // Packages to scan
+        // Packages to scan - only public API, not internal implementation
         final var packages = List.of(
-            "jdk.sandbox.java.util.json",
-            "jdk.sandbox.internal.util.json"
+            "jdk.sandbox.java.util.json"
         );
 
         final var classLoader = Thread.currentThread().getContextClassLoader();

json-java21-api-tracker/src/test/java/io/github/simbo1905/tracker/ApiTrackerTest.java

@@ -50,10 +50,10 @@ void testDiscoverLocalJsonClasses() {
                     "jdk.sandbox.java.util.json.JsonNull"
                 );
 
-            // Should also find internal implementation classes
+            // Should NOT find internal implementation classes (public API only)
             assertThat(classes.stream().anyMatch(c -> c.getName().startsWith("jdk.sandbox.internal.util.json")))
-                .as("Should find internal implementation classes")
-                .isTrue();
+                .as("Should not find internal implementation classes - public API only")
+                .isFalse();
 
             // Should be sorted
             final var names = classes.stream().map(Class::getName).toList();