test(json): add Json and Simple round-trip coverage

[GaspardKirira]

Summary

Add comprehensive round-trip tests between the main JSON model and the Simple value model.

Current behavior

Tests cover conversion from Simple values to Json.

There is no complete round-trip suite validating semantic preservation across nested values.

Expected behavior

Supported values should preserve their meaning through:

• Simple → Json → Simple;
• Json → Simple → Json.

The test suite must clearly identify values that cannot be represented losslessly.

Scope

• Test null round trips.
• Test boolean round trips.
• Test signed integer round trips.
• Test floating-point round trips.
• Test string round trips.
• Test nested array round trips.
• Test nested object round trips.
• Test empty arrays and objects.
• Test property ordering.
• Test unsupported unsigned ranges.
• Test recursion limits.

Suggested files

• modules/json/tests/json_simple_roundtrip_test.cpp
• modules/json/include/vix/json/convert.hpp

Acceptance criteria

• Supported primitive values round-trip.
• Nested arrays round-trip.
• Nested objects round-trip.
• Empty structures preserve their type.
• Unsupported values fail with documented diagnostics.
• No silent numeric narrowing occurs.
• Existing conversion tests continue to pass.

Non-goals

This issue does not include:

• byte-for-byte serialized-text equality;
• arbitrary-precision numbers;
• duplicate object keys;
• JSON schema equivalence.