Fix invalid JSON from unescaped backslashes in MQL5 string serialization

[biohazardxxx]

Summary

• JSONString.toString() in mq5/json.mqh emitted raw strings without escaping special characters. When MQL5 functions returned Windows paths (e.g. TerminalInfoString), unescaped backslashes produced invalid JSON, causing JsonReaderException: Bad JSON escape sequence on the C# client side.
• Added proper JSON escaping for backslash, double-quote, newline, carriage return, and tab characters during serialization.
• The MQL4 json.mqh has the same latent bug but is worked around by a manual StringReplace in Execute_TerminalInfoString; this fix addresses it at the serialization layer for MQL5.

Test plan

• Compile MtApi5.mq5 with MetaEditor to verify MQL5 compilation succeeds
• Connect a .NET client and call TerminalInfoString with path-returning properties (e.g. TERMINAL_PATH, TERMINAL_DATA_PATH) — verify valid JSON is returned and parsed without exceptions
• Verify strings containing quotes, newlines, and tabs are also correctly escaped in JSON output

🤖 Generated with Claude Code

[tudstudent]

Fix looks correct — we verified it against a live MT5 terminal (TerminalInfoString with TERMINAL_PATH now returns valid JSON with escaped backslashes).

Note: mq4/json.mqh has the same toString() bug but is worked around by a manual StringReplace in Execute_TerminalInfoString. This PR fixes it at the serialization layer for MQL5, which is the right approach. Consider applying the same fix to mq4/json.mqh for consistency — any new MQL4 function returning strings with backslashes/quotes/newlines would hit the same issue.

[tudstudent]

For reference: our PR #297 includes this JSON escaping fix for MQL5 plus the equivalent fix needed in MQL4's json.mqh, along with 6 other bug fixes. If you'd prefer a single merge that covers everything, #297 is the combined option.