Optimize logger string formatting to avoid repeated allocations

[Copilot]

Addresses feedback on PR #668 to eliminate inefficient string operations in the logger's placeholder replacement logic.

Changes

• Removed repeated find() and replace() calls that allocated new buffers on each iteration
• Build output directly into stringstream by iterating through header_ once, appending arguments when "%@" placeholders are encountered

Before

std::string formattedHeader = header_;
while ((pos = formattedHeader.find("%@", pos)) != std::string::npos) {
  formattedHeader.replace(pos, 2, argStrings[argIndex]);  // allocates + copies
  pos += argStrings[argIndex].length();
  ++argIndex;
}
ss << formattedHeader;

After

for (size_t i = 0; i < header_.size(); ++i) {
  if (i + 1 < header_.size() && header_[i] == '%' && header_[i + 1] == '@' &&
      argIndex < argStrings.size()) {
    ss << argStrings[argIndex];
    ++argIndex;
    ++i;
  } else {
    ss << header_[i];
  }
}

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