feat: report ready/init_error on startup via JSON-RPC notification

Why:
When the deltachat-rpc-server encounters a fatal error during early startup
(e.g., when the accounts directory is invalid, a file instead of a dir, or
otherwise inaccessible), it exits. The Python RPC client previously lacked
a structured way to wait for the server to be fully initialized or to
detect early startup failures gracefully. This led to hanging tests or
obscure broken pipe errors rather than clear initialization errors.

How:
- The RPC server now sends a JSON-RPC notification on stdout at startup:
  - "ready" with core_version, server_path, and accounts_dir on success
  - "init_error" with error message if accounts directory initialization fails
- The Python RPC client reads the first line from stdout to ensure the server is ready.
- The Python client raises JsonRpcError on init_error, enabling early
  failure detection and fast-failing rather than stalling.
- Added tests to ensure the client fails immediately on invalid dirs.

deltachat-rpc-client/README.md

@@ -2,6 +2,9 @@
 
 RPC client connects to standalone Delta Chat RPC server `deltachat-rpc-server`
 and provides asynchronous interface to it.
+`rpc.start()` blocks until the server is initialized
+and will raise an error if initialization fails
+(e.g. if the accounts directory could not be used).
 
 ## Getting started