SDK: a process started with --await-claim refuses messages that arrive before it is claimed

In print or SDK mode (Claude Code driven by another program), a process started with --await-claim waits to be claimed before it works. A message that arrives during that wait is now answered with a not_claimed error instead of being run. The error comes in two forms:

• one for when the claim failed
• one for when no claim has arrived yet, telling the sender to send claim_session before the first message

The first message added by the host program itself is tracked separately. If input ends before it is handled, it is closed with "the input ended first".