test(runtime): expand JsonValue parser/renderer unit tests

[nankingjing]

What

Expands unit-test coverage for the hand-rolled JSON value type in rust/crates/runtime/src/json.rs.

Before this change the module had only two tests (renders_and_parses_json_values, escapes_control_characters) despite implementing a full self-contained JsonValue parser, renderer, accessor set, and error type. This PR keeps both existing tests and adds eleven focused ones (13 total).

Coverage added

• Rendering – null/bool/number/string primitives, empty/populated arrays, and objects (verifying BTreeMap sorted-key determinism).
• String escaping – double-quote and backslash escaping, the dedicated \r short escape, and the \uXXXX fallback for other control characters (U+0001).
• Parsing – primitives, signed integers, surrounding-whitespace tolerance, and nested arrays/objects (with accessor drill-down).
• Round-tripping – parse(render(v)) == v for a mixed object containing a string, negative number, and an array of bool/null.
• Error paths – empty input, truncated literal, trailing content, unterminated string, unterminated array, and i64 overflow rejection.
• Accessors – as_* helpers return None on variant mismatch.
• Error type – JsonError Display renders its message.

Notes

• Pure unit tests only; no production code changed and no new dependencies.
• Assertions were derived directly from the current implementation's behavior (e.g. the parser is integer-only via i64, objects render with sorted keys because they are backed by BTreeMap).
• Follows the existing test style in the module (use super::{...}, .expect(...), assert!/assert_eq!).

[1716775457damn]

Nice coverage — the round-trip tests (parse→render→parse) are the right way to validate parser/renderer consistency. Good to see edge cases like empty structs, null in arrays, and nested objects covered. The pre_rfc_parse→
ender path is important since that's the hot loop in production.

[nankingjing]

Thanks for the review! The round-trip test was the piece I most wanted in place — asserting that anything the renderer produces re-parses to the identical value pins parser/renderer consistency as a single property instead of a pile of one-directional assertions. The edge cases (empty structs, null inside arrays, i64 boundary values, malformed input) target the shapes production JSON actually hits on that hot path. Happy to extend coverage if you spot any gaps.