HTTP mock: multiple multipart files per name and repeated request headers #138

Split from #110. Line numbers refer to branch chore/php-8.4-dependabot (#98). Verified = reproduced.

• Two files under the same multipart name (e.g. files[]) keep only the last one, because parts are keyed by name (RealRequestFactory.php:118). Supporting several parts per name changes the structure of RealRequest::getMultiparts() (one entry per name today), which custom callbacks and code that constructs RealRequest rely on. Verified.
• Repeated request headers (['X-Multi' => ['a', 'b']]) keep only the last value (RealRequestFactory.php:53-55). They should be combined (a, b) or matchable individually. Verified.

Breaking: mocks that match the last value of a repeated header (header('X-Multi', 'b')) stop matching, and the multipart structure of RealRequest changes.