Fix warnings in non-unity static-analysis builds

[bozalina]

Description of Changes

Headers:

• Add override to defaulted destructors in ResponseMessage and 14 WebSocket request/response subclasses (-Wsuggest-override).
• Cast size_t loop indices to int in LogParameters::operator== to match getLogPath's int parameter.

Sources:

• Combiner.cpp: fold insertSuccess local into assert(pair.second) directly; the local was unused under NDEBUG.
• JsonHelper.cpp / HttpClient.cpp / AwsSigV4Utility.cpp: static_cast
  on size_t values passed to APIs that take int (send, HMAC_Init_ex, getLogPath).
• HttpClient.cpp: widen jsonStart/jsonEnd from int to size_t to match std::string::find return type.
• WebSocketppClientWrapper.cpp: mark unused OnMessage/OnTlsInit parameters as (void), drop an unused local errorMessage.
• GameLiftServerState.cpp: initialize m_onUpdateGameSession(nullptr) in both GAMELIFT_USE_STD and non-STD constructor init lists to silence uninitialized-member warnings.

Description of Testing and Validations of Changes

Verified by building the vendored SDK sources as part of the GameLift Server SDK plugin inside an Unreal Engine 5 project (MSVC, non-unity, warnings-as-errors), which compiles cleanly with these changes.

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Checklist

• [ X ] I have ensured that the Server SDK builds successfully and that all unit tests pass.
• [ X ] I have updated any affected documentation to reflect this change.
• [ X ] I have added unit tests for any new code or logic introduced.
• [ X ] I have linted my code before submitting this PR.

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