fix(runtime): force tool-free child summaries

[likun666661]

Summary

• force toolChoice: none whenever the runtime has an authoritative empty active-tool set
• prevent DeepSeek-compatible child finalization steps from emitting native DSML tool-call envelopes as plain summary text
• lock the child finalization contract with a regression assertion

Real-model verification

Reproduced before the fix with a local DeepSeek chain:

• root: deepseek-v4-pro
• configured subagent preset: deepseek-flash-reader
• two swarm children: deepseek-v4-flash
• maxSteps=12

Before the change, both child summaries ended as raw DSML tool-call envelopes and the root had to recover through agent_output.

After the change:

• root session: fd8c6c3e-e944-4296-9d3d-fade5f18e2c1
• root called only load_tools, agent_list, and agent_swarm
• both child sessions persisted deepseek-v4-flash, deepseek-flash-reader, and local_read
• both children read their assigned runtime source file
• both swarm summaries were complete natural-language results with no DSML envelope
• root synthesized directly from the swarm result without agent_output

Tests

• npm --workspace @maka/runtime run typecheck
• npm --workspace @maka/runtime run build
• targeted child-finalization regression test
• full ai-sdk-backend test file: 175 passed
• real DeepSeek Pro to Flash swarm E2E: passed
• full runtime suite: 2740 passed, 5 failed, 9 skipped; all 5 failures are pre-existing macOS /var vs /private/var path-canonicalization assertions in builtin-tools.test, unrelated to this change