Support typed JSON values in transaction metadata

[SDILogin]

Summary

TransactionPayload.metadata was Map<String, String>. The MetadataEntryAdapter always wrapped every value through encodeJsonString(), producing JSON strings on the wire. Iroha's executor rejects transactions whose gas_limit is a JSON string ("1000") — it requires a JSON number (1000).

Changes

• New JsonValue inline value class (core/model/JsonValue.kt) — wraps a raw JSON literal with factory methods: string() (quotes + escapes), number() (bare digits), bool(), raw()
• TransactionPayload.metadata type changed from Map<String, String> to Map<String, JsonValue>
• TransactionPayloadAdapter — replaced JsonAdapter/JsonStringAdapter with JsonValueFieldAdapter that writes rawJson directly through STRING_ADAPTER. Removed encodeJsonString, decodeJsonString, parseJsonString, hexNibble, HEX_DIGITS (all now unused)
• Tests updated to use JsonValue.string() / JsonValue.number() where metadata is constructed

Wire format

String values produce identical bytes — JsonValue.string("hello").rawJson is "hello" (with quotes), same as the old encodeJsonString("hello"). Number values like JsonValue.number(1000) emit 1000 (4 bytes, no quotes) instead of the old "1000" (6 bytes with quotes).

Usage

val payload = TransactionPayload(
    // ...
    metadata = mapOf(
        "gas_asset_id" to JsonValue.string(gasAssetId),
        "gas_limit"    to JsonValue.number(1000),
    ),
)