Fix critical error handling and resource management in C++ code

[jvmvik]

Summary

This PR addresses critical error handling, resource management, and thread safety issues in the C++ client library. All changes are backwards compatible.

Critical Fixes

• Null pointer checks: Validate callback inputs and CURL handle initialization
• Integer overflow prevention: Safe size calculations in callback to prevent buffer issues
• CURL error handling: Check curl_easy_perform() return value and report errors
• JSON parsing validation: Use HasParseError() to detect and signal parsing failures
• Memory allocation checks: Validate curl_easy_escape() allocations
• Thread safety: Replace static bool with std::call_once for CURL initialization

Code Quality Improvements

• Remove using namespace directives (best practice)
• Use std::ostringstream for efficient string building
• Proper cleanup of CURL resources on all error paths
• Exception safety in callback function
• Meaningful error messages instead of silent failures

API Additions

• setTimeout(int seconds) method for configurable request timeouts

Test plan

• Build project successfully with new code
• Verify error handling with network failures
• Test timeout configuration
• Verify thread safety with concurrent requests (if applicable)

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