context: add ContextSetter.size() (pure-Java slot count)

The dd-trace-java Phase 2 bridge sizes its app-offset arrays from the context
attribute count. It previously read that as snapshotTags().length — a DBB read
that creates a ThreadContext. Expose size() (attributes.size(), pure Java) so the
bridge can size without touching the DBB path; offsetOf + size are the only
ContextSetter methods it still uses.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rkennke

ddprof-lib/src/main/java/com/datadoghq/profiler/ContextSetter.java

@@ -69,6 +69,11 @@ public int offsetOf(String attribute) {
         return attributes.indexOf(attribute);
     }
 
+    /** Number of (deduplicated, truncated) context attribute slots. Pure Java; no native/DBB read. */
+    public int size() {
+        return attributes.size();
+    }
+
     public boolean setContextValue(String attribute, String value) {
         return setContextValue(offsetOf(attribute), value);
     }