Fix cookie parsing in MockMvcClientHttpRequestFactory - #37685

When using RestTestClient with MockMvc, multiple cookies are incorrectly parsed as a single cookie.

For example, sending Cookie: foo=bar; baz=qux to the following controller results in request.getCookies() returning only one cookie named foo with the value bar; baz=qux, instead of two separate cookies:

@GetMapping(path = "/cookies")
public String cookies(HttpServletRequest request) { 
	Cookie[] cookies = request.getCookies();
	
	// ...
}

This happens because MockMvcClientHttpRequestFactory splits Cookie headers on commas(,).

This PR fixes the issue by splitting Cookie headers on semicolons(;) instead of commas(,), ensuring that each cookie is parsed separately.