Expand RequestContext.isAjax() to recognize Fetch Metadata requests

[Copilot]

event.isAjax() only recognized X-Requested-With: XMLHttpRequest, so modern fetch() calls were misclassified as non-AJAX. This caused downstream behavior (e.g., debugger rendering) to run on requests that should be treated as async/AJAX.

• Request classification update

  • Kept legacy XHR detection.
  • Added Fetch Metadata detection using request headers:
    • Sec-Fetch-Dest == "empty"
    • Sec-Fetch-Mode != "navigate"
  • This classifies typical fetch() API calls as AJAX while excluding browser navigation requests.

• Behavioral coverage added

  • Added focused specs for:
    • XHR header path (XMLHttpRequest) → true
    • Fetch Metadata path (cors + empty) → true
    • Navigation request path (navigate + document) → false

boolean function isAjax(){
	var xRequestedWith = getHTTPHeader( "X-Requested-With", "" );
	var fetchMode      = getHTTPHeader( "Sec-Fetch-Mode", "" );
	var fetchDest      = getHTTPHeader( "Sec-Fetch-Dest", "" );

	return (
		xRequestedWith eq "XMLHttpRequest" ||
		( len( fetchMode ) && fetchMode neq "navigate" && fetchDest eq "empty" )
	);
}