CVE-2026-48512: MessagePack-CSharp: JSON conversion APIs can recurse without consistent depth enforcement

MessagePack-CSharp’s JSON conversion helpers contain multiple recursion paths that do not consistently enforce a depth limit. These paths are in the JSON conversion component rather than normal typed MessagePack deserialization.

Three related issues are covered by this advisory:

1. MessagePackSerializer.ConvertFromJson recursively processes nested JSON arrays and objects in FromJsonCore() without consulting MessagePackSecurity.MaximumObjectGraphDepth.
2. TinyJsonReader.ReadNextToken() recursively consumes comma and colon separator characters, allowing even malformed JSON with long separator runs to consume one stack frame per character.
3. MessagePackSerializer.ConvertToJson applies depth checks to arrays and maps, but the typeless extension branch for ext-100 recursively calls ToJsonCore() without applying MessagePackSecurity.DepthStep(ref reader).

Each path can allow attacker-controlled input to exhaust the process stack and trigger an uncatchable StackOverflowException instead of failing with a catchable parse or serialization exception.