The Messages stream failed every max_tokens stop with MaxTokensTruncation,
so a response that had already emitted complete tool calls before the
output budget ran out lost those calls and failed the turn. The stream
now completes with StopReason::ToolCalls when every collected call's
arguments parse (or are empty, the zero-argument convention); a cut call
or a text-only truncation still fails so compact-on-error keeps working.
The ChatCompletions and Responses streams already behaved this way.
Ported from upstream 1.0.1x LengthPolicy::CompleteToolCalls, reshaped
into the stream layer because Kigi has no caller that chooses a policy.
Claude-Session: https://claude.ai/code/session_01RUqV8HVGXcAETyYbZ2tPQ4
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