fix(partykit): preserve delete tombstones

[clentfort]

fix(partykit): preserve delete tombstones

Encode PartyKit messages with undefined-aware JSON and decode them without using a reviver so tombstone keys are preserved. This keeps delete changes intact across transport instead of dropping row and cell deletions during parse.

Fixes: #281

[clentfort]

Small REPL proof for why a reviver cannot preserve delete tombstones:

const UNDEFINED = '\uFFFC';

const jsonParseWithUndefinedOld = (str) =>
  JSON.parse(str, (_key, value) => (value === UNDEFINED ? undefined : value));

const replaceUndefinedString = (value) => {
  if (value === UNDEFINED) {
    return undefined;
  }
  if (Array.isArray(value)) {
    return value.map(replaceUndefinedString);
  }
  if (value && typeof value === 'object') {
    for (const key of Object.keys(value)) {
      value[key] = replaceUndefinedString(value[key]);
    }
  }
  return value;
};

const jsonParseWithUndefinedNew = (str) =>
  replaceUndefinedString(JSON.parse(str));

const serializedChanges =
  '{"feeding-sessions":{"1767445910545":"\\uFFFC"}}';

const parsedOld = jsonParseWithUndefinedOld(serializedChanges);
const parsedNew = jsonParseWithUndefinedNew(serializedChanges);

console.log('parsedOld:', parsedOld);
console.log(
  'old keeps row key:',
  '1767445910545' in parsedOld['feeding-sessions'],
);

console.log('parsedNew:', parsedNew);
console.log(
  'new keeps row key:',
  '1767445910545' in parsedNew['feeding-sessions'],
);
console.log(
  'new row value is undefined:',
  parsedNew['feeding-sessions']['1767445910545'] === undefined,
);

Expected output:

• old keeps row key: false
• new keeps row key: true
• new row value is undefined: true

[jamesgpearce]

Amazing - thank you!