Add per-session code-mode execution limits

[copyberry]

Add per-session code-mode execution limits

What changed

• Add create_session_with_limits and session-scoped cell execution limits.
• Clamp execute and wait yield times to the session's max_yield_time_ms
  without terminating the running cell.
• Negotiate support with remote code-mode hosts and include non-default limits
  in session/open, while keeping unlimited sessions compatible with hosts and
  providers that do not support limits.

Testing

• Cover yield-time clamping, zero-timeout behavior, and isolation between
  sessions.
• Cover wire serialization, capability negotiation, unsupported hosts, and
  shared process-host execution.

[github-actions]

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