feat(base-transport-json): replace Jackson with base-json

[deer]

Migrates base-transport-json off the Jackson streaming API (JsonGenerator/JsonParser) and onto the base-json typed value model introduced in #54. The Codec interface gains encode/decode methods that operate on JsonValue instead of void write/read methods that mutated a streaming generator/parser, all codec implementations simplify dramatically as a result, and JsonTransport itself drops its requires com.fasterxml.jackson.core module dependency entirely. Jackson is removed from the root BOM. Checked IOException is replaced with unchecked IllegalStateException throughout, since codec failures are programming errors rather than I/O failures. The polymorphic type discriminator field is renamed from "type" to "@type" to prevent silent key collisions with marshalled objects that have a field of the same name.