Fix std::isspace undefined behavior with unsigned char cast

[Copilot]

std::isspace has undefined behavior when passed negative char values, which can occur on platforms where char is signed and characters have values > 127.

Changes

• Cast char to unsigned char before passing to std::isspace in GatewaySupervisor.cc
• Add #include <cctype> for explicit std::isspace declaration

Example

// Before: undefined behavior for negative signed chars
if(varName[0] == '_' || std::isspace(varName[0]) || ...)

// After: safe for all char values [0, 255]
if(varName[0] == '_' || std::isspace(static_cast<unsigned char>(varName[0])) || ...)

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