fix(mcp): answer JSON-RPC batches for protocol 2025-03-26

Problem

portless mcp accepts protocol version 2025-03-26. That version requires servers to accept JSON-RPC batches. handleLine dropped any JSON array, so a client waiting on those request ids never got a response.

Changes

handleLine treats a JSON array as a batch:

• [] returns one -32600 Invalid Request object with id null.
• A non-object entry gets that same -32600 object inside the response array.
• An object entry goes through handleRequest. Notifications are omitted.
• A notification-only batch returns null, so nothing is written.
• Any other batch returns one JSON array, in request order. A one-request batch stays a one-element array.

Single JSON-RPC objects are unchanged, and initialize still accepts 2025-03-26.

2025-06-18 removed batching. A batch response is written only when the client sent a batch, so clients that send one object per line see the same stdout as before.

Fixes #519

Unit tests cover each batch case, and a stdio test runs the three-line script from #519. pnpm type-check, pnpm lint, and the package test suite passed.