[GIT-PULLS] [php-src] PR #22771: ext/soap: truncate a user_agent containing newline characters

Pull Request: https://github.com/php/php-src/pull/22771
Author: iliaal

GH-17976 (php/php-src#21658) sanitized `user_agent` where the HTTP wrapper emits headers and left the SOAP client alone, so CRLF in the value still injects request headers. Verified against a mock server on master: `ini_set('user_agent', "ua\r\nX-Injected: yes")` puts a real `X-Injected: yes` on the wire. A NUL-prefixed value (`"ua\0valid\r\n..."`) injects too now that 21658 made the ini global a zend_string, where the old char* form truncated at the NUL.

Three sources feed that sink unchecked (the SoapClient `user_agent` option, the `http.user_agent` context option, and the ini), so this resolves them to one zend_string and emits through the same truncate-and-warn check the HTTP wrapper uses. The two user-zval sources were already length-aware and injected before 21658, so 8.4 and 8.5 are affected as well.

The helper mirrors `smart_str_append_header_value()` rather than exporting it from ext/standard, which would add API surface there for a soap fix; the duplication keeps behaviour identical for the same ini across both clients.