[arifBurakDemiray]

Card C-137: Bring R132 Modifier mul to the C++ SDK

Why: the C++ SDK cannot multiply a custom user property on the server ($mul), which Android, iOS, HarmonyOS and React Native ship (parity R132, reference ModuleUserProfile.multiply).
Change:

• New Countly::multiplyCustomUserDetail(const std::string &key, double value) queues user_details={"custom":{"<key>":{"$mul":<value>}}}, after flushing recorded events like setCustomUserDetails.
• Applies the SBS user property filter to the key as given, then the max key length. A call before start(), an empty key and a non-finite value are ignored with a log line.
• Details set before start() that still wait for the first session are queued first, so the server applies them before the multiplication.
• setCustomUserDetails and addEvent share the new filter helper (same behaviour).
  Decided:
• One request per call, named after setCustomUserDetails; rejected a userProfile() object with save(), since this SDK has no profile cache.
• double, like iOS (NSNumber), HarmonyOS, React Native and Dart (Add multiply user property modifier countly-sdk-dart#13). The server takes JSON numbers, so 2.0 lands as 2 (plugins/users/api/ingestor.js).
• Before start() the call is ignored, like addEvent: Android's userProfile() does not exist before init either.
  Checked:
• tests/user_details.cpp: 6 cases, 67 assertions, and all 6 fail with the method body stubbed out. Whole suite on macOS arm64: 77 cases, 1300 assertions (SQLite off) and 94 cases, 1640 assertions (SQLite on).
• Mock Countly server over real HTTP (libcurl, /i and /o/sdk): 7 /i requests in the expected order (events, waiting details, four $mul, begin_session without details). The server's SBS blacklist and lkl applied, a key with &=+% arrived intact, 3 ignored calls logged.
• Live on master.count.ly (desktop test app, fresh device): three properties set before start(), then multiplied. The profile shows 10 × 2 = 20, 7 × 0.5 = 3.5 and 3 × 3 = 9 (the &=+% key), and all 6 requests were answered Success in order.
• Reviews: code review, simplify, security review, no open findings. CI: all 44 jobs green on 2373783.
  Not checked: the C++ help-center page has no "Modifying data" section yet (countly-backup-documentation, its own docs card).