fix(sdk): Add proper BigInt serialization for JSON API responses

[Vincent6581]

Summary

JSON.stringify throws TypeError: Do not know how to serialize a BigInt. This adds first-class BigInt ⇄ JSON support, per issue #193.

Acceptance criteria

Criterion	Status
BigInt serialized as strings in all JSON responses	✅ bigIntReplacer / serializeBigInts() render every bigint as a decimal string
Deserialization parses strings back to BigInt	✅ bigIntReviver (tagged) and createBigIntReviver(keys) (plain strings by field name)
Custom JSON replacer/reviver exported	✅ bigIntReplacer, taggedBigIntReplacer, bigIntReviver, createBigIntReviver
Consistent across all client classes	✅ StellarRWASDK.toJSONSafe() / .stringifyJSON() / .parseJSON() — one path all client results flow through

Two modes

Plain-string — for external APIs where big integers are strings:

import { bigIntReplacer, serializeBigInts } from 'stellar-rwa-sdk';

JSON.stringify({ amount: 123n, fees: [1n, 2n] }, bigIntReplacer);
// {"amount":"123","fees":["1","2"]}

serializeBigInts(portfolio); // deep copy, bigint -> string, handles nested objects/arrays/Map/Set

Tagged (lossless round-trip) — for caches / persistence:

import { stringifyJSON, parseJSON } from 'stellar-rwa-sdk';

const json = stringifyJSON({ amount: 123n, nested: { x: [5n] } });
// {"amount":{"$bigint":"123"},"nested":{"x":[{"$bigint":"5"}]}}

const back = parseJSON(json);
typeof back.amount;        // "bigint"
back.nested.x[0] === 5n;   // true

Round-trips correctly through nested objects and arrays (negative values and values beyond Number.MAX_SAFE_INTEGER included).

Restoring from plain strings when you know the schema

import { createBigIntReviver } from 'stellar-rwa-sdk';

JSON.parse(body, createBigIntReviver(['amount', 'totalSupply', 'perTokenAmount']));

Files changed

• sdk/src/types.ts — replacers, revivers, serializeBigInts, stringifyJSON / parseJSON, BIGINT_TAG / TaggedBigInt / isTaggedBigInt
• sdk/src/index.ts — re-exports + StellarRWASDK.toJSONSafe() / .stringifyJSON() / .parseJSON()

Closes #193

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[Kevin737866]

@Vincent6581 conflict