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!).