Fix jet cgi=yes CGI variable loading and related bugs

- __jet.cpp: the cgi keyword check compared against "true" but the
  documented/intended value is "yes", so <jet cgi="yes" ...> never
  actually turned CGI mode on; fixed. Also replaced the hardcoded
  fake multipart boundary ("xxxx") with real extraction from the
  Content-Type header's boundary= parameter.
- Global.h: default-initialize cgiData to NULL (a GET request with
  cgi="yes" never allocates it, so it was previously left
  dangling); Global.cpp now sets the cookie jar's delimiter to ";"
  to match RFC 6265, since it defaulted to CGIFormattedData's
  &-based form-data delimiter.
- Global.cpp: fix addSession()'s missing throw that let it silently
  continue past a duplicate session id.
- Tag.cpp: fix the CGI-variable lookup branch, which failed to
  compile (cgiData is a pointer; needed to dereference it before
  indexing) and added a null check so a CGI request that never
  populated cgiData (bad Content-Type, or the null-safety gap
  above) throws a clear error instead of crashing; implemented the
  previously-empty session-variable lookup branch; left the
  unreachable duplicate environment-variable branch as-is with a
  comment, since fixing its sigil is a user-facing template-syntax
  decision, not a bug fix.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

__jet.cpp

@@ -10,7 +10,7 @@ namespace jet {
    
    __jet::__jet(coreutils::ZString &in, coreutils::MString &parentOut, Global &global, Tag *parent, Tag *local) : Tag(in, parentOut, global, parent, this) {
       char *cookies;      
-      if(keywordDefined("cgi") && (resolveKeyword("cgi") == "true")) {
+      if(keywordDefined("cgi") && (resolveKeyword("cgi") == "yes")) {
 	 global.cgi = true;
 	 
 	 if(keywordDefined("sessiondir")) {
@@ -35,10 +35,14 @@ namespace jet {
 	 if(requestMethod == "POST") {
 	    coreutils::ZString contentLength(getenv("CONTENT_LENGTH"));
 	    coreutils::ZString contentType(getenv("CONTENT_TYPE"));
-		ZString boundary = "xxxx";
-	    
-	    global.cgiData = new coreutils::CGIFormData(0, contentType, contentLength.asInteger(), boundary);	      
-	 }	
+	    coreutils::ZString boundary;
+	    int boundaryPos = contentType.find("boundary=");
+	    if(boundaryPos != -1) {
+	       boundary = contentType.substring(boundaryPos + 9);
+	    }
+
+	    global.cgiData = new coreutils::CGIFormData(0, contentType, contentLength.asInteger(), boundary);
+	 }
       }
       processContainer(container, NULL, true);
    }