test(yet_another_json_isolate): add full coverage test suite

[spydon]

What

Expands the test suite for yet_another_json_isolate from 4 tests to a comprehensive suite covering all reachable code paths.

Line coverage of lib/src/_isolates_io.dart goes from 49/63 to 59/63. The only remaining uncovered lines are the RemoteError (native error) branch in _handleRes, which is defensive code that is unreachable through the public API: it only fires when a non-bool method flag reaches the isolate, and the public decode/encode methods always send [input, bool].

Coverage added

• Initialization: both manual initialize() and lazy first-call initialization, for decode and encode.
• Data types: top-level lists, nested structures, ints/doubles/booleans/null/strings, unicode, bare JSON primitives, and encode/decode round trips.
• Error handling: FormatException on invalid JSON, JsonUnsupportedObjectError on non-encodable objects, and verification that the isolate stays usable after a decode or encode error.
• Concurrency and ordering: concurrent decodes, interleaved encodes/decodes, and many sequential operations all resolve to the correct results in order.
• Lifecycle: dispose on a lazily initialized isolate, and two isolates operating independently.

Two io-specific behaviors (debugName getter, and the double-initialize() assertion) live in a separate @TestOn('vm') file so the shared suite compiles and passes on both the VM and web (dart2js/chrome) targets.

Verification

• dart test (VM): all pass
• dart test -p chrome (web): all pass
• dart analyze: no issues
• dart format: clean