Bug(expr): string(list[string]) fails to escape an embedded double quote, producing invalid JSON

[uberware]

Summary

RFC 0006 specifies string() on a list produces "the JSON string representation".
string(['a"b']) returns ["a"b"] — the embedded " is not escaped as \", so the output
is not valid JSON: the third " closes the string early.

Versions

• openjd-model 0.11.1 and 0.11.2 from PyPI
• openjd-rs main at 1a89f3a — still reproduces

Reproduction

import json
from openjd.expr import parse_expression
r = parse_expression("string(['a\"b'])").evaluate_with_metrics()
out = str(r.value)
print(out)               # -> ["a"b"]
json.loads(out)          # -> json.decoder.JSONDecodeError:
                         #    Expecting ',' delimiter: line 1 column 5 (char 4)

Why this is a bug

The output is consumed as JSON by construction ("the JSON string representation"), and this
one is rejected by any JSON parser. Note repr_json handles the same input correctly — the
escaping gap is specific to string()'s list path.