fix(mcp): propagate a failed remote initialized notification (#432)

The remote (Streamable HTTP) handshake discarded every error from posting
notifications/initialized with `try?` and carried straight on to tool
discovery. The stdio transport propagates the same failure with a plain `try`.

So the two transports disagreed about whether initialization has to succeed, and
a remote server that rejected the notification was still attached and its tools
still offered to the model -- against a server that had just said it was not
ready to serve them.

One character, and the two transports now agree.

Diff taken from candidate PR #440.

Closes #432

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011ccLBbEaVVd4sJyUd5wyhA