Fix json schema with '\' in literals

[i-v-s]

Fixes issue Misc. bug: Symbol '' is not escaped in the json schema literals.

[ehoogeveen-medweb]

Depending on the implementation, there might be an ordering issue here: If " is escaped to \" and then \ is escaped to \\, " might get escaped to \\". Generally \ should be escaped first to avoid such issues - but I don't know if it's a problem here.

[i-v-s]

I think that twice replacement is not possible here, because the substitution result does not participates in the matching. My test shows that all works as expected:

$ ./debug/bin/llama-cli -m gemma-3-4B-it-Q8_0.gguf --json-schema '{"properties":{"code":{"const":"auto s = \"000\";","description":"Generated code","title":"Code","type":"string"}},"required":["code"],"title":"DecoderResponse","type":"object"}'

...

== Running in interactive mode. ==
 - Press Ctrl+C to interject at any time.
 - Press Return to return control to the AI.
 - To return control without starting a new line, end your input with '/'.
 - If you want to submit another line, end your input with '\'.
 - Not using system message. To change it, set a different value via -sys PROMPT


> AA
{"code":"auto s = \"000\";"}

[CISC]

Please add a test to tests/test-json-schema-to-grammar.cpp.