Reject ';' as a dd tracestate key character

isAllowedKeyChar didn't exclude the element separator, so a key scan starting right after an empty element
(e.g. "dd=s:2;;o:some") would swallow the next ';' into the key instead of failing, silently dropping the
origin tag rather than rejecting the malformed member.

Datadog's sibling codec already excludes its element separator from key chars; align W3C's the same way.

Flagged by Copilot review on PR #12229.

Author: mcculls

dd-trace-core/src/main/java/datadog/trace/core/propagation/ptags/W3CPTagsCodec.java

@@ -365,7 +365,10 @@ private static int validateCharsUntilSeparatorOrEnd(
   private static boolean isAllowedKeyChar(int c) {
     // We already know that the segments have been validated against the valid chars for
     // the general tracestate header
-    return c > MIN_ALLOWED_CHAR && c <= MAX_ALLOWED_CHAR && c != KEY_VALUE_SEPARATOR;
+    return c > MIN_ALLOWED_CHAR
+        && c <= MAX_ALLOWED_CHAR
+        && c != KEY_VALUE_SEPARATOR
+        && c != ELEMENT_SEPARATOR;
   }
 
   private static boolean isAllowedValueChar(int c) {

dd-trace-core/src/test/java/datadog/trace/core/propagation/W3CHttpExtractorTest.java

@@ -115,6 +115,8 @@ void checkMaxFromW3CTraceIds(@ConvertWith(TraceIdTestConverter.class) DDTraceId
     "keep with trailing element separator          | '00-00000000000000000000000000000001-123456789abcdef0-01' | 'dd=s:2;o:some;'         | USER_KEEP    |                           | some  ",
     "keep with trailing separator and OWS          | '00-00000000000000000000000000000001-123456789abcdef0-01' | 'dd=s:2;o:some; \t'      | USER_KEEP    |                           | some  ",
     "keep with trailing separator OWS before comma | '00-00000000000000000000000000000001-123456789abcdef0-01' | 'dd=s:2;o:some;  ,x=y'   | USER_KEEP    |                           | some  ",
+    "reject dd member with empty element in middle | '00-00000000000000000000000000000001-123456789abcdef0-01' | 'dd=s:2;;o:some'         | SAMPLER_KEEP | SamplingMechanism.DEFAULT |       ",
+    "reject dd member with leading separator       | '00-00000000000000000000000000000001-123456789abcdef0-01' | 'dd=;s:2;o:some'         | SAMPLER_KEEP | SamplingMechanism.DEFAULT |       ",
     "keep with user keep state and manual dm       | '00-00000000000000000000000000000001-123456789abcdef0-01' | 'dd=s:2;o:some;t.dm:-4'  | USER_KEEP    | SamplingMechanism.MANUAL  | some  ",
     "drop with user keep state and manual dm       | '00-00000000000000000000000000000001-123456789abcdef0-00' | 'dd=s:2;o:some;t.dm:-4'  | SAMPLER_DROP |                           | some  ",
     "drop with user drop state                     | '00-00000000000000000000000000000001-123456789abcdef0-00' | 'dd=s:-1;o:some'         | USER_DROP    |                           | some  ",