fix(async-toolset): use underscore in synthetic call_id

[longcw]

Summary

• Replace / with _ in the synthetic call_ids generated for ctx.update(...) and the final return in AsyncToolset.
• The previous format violated the ^[a-zA-Z0-9_-]+$ pattern enforced by some realtime backends, causing tool outputs to be rejected and never delivered back to the agent.

Closes #5652

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.

[toubatbrian]

This is an automated Claude Code Routine created by @toubatbrian. Right now it is in experimentation stage. The automation will start porting this PR into agents-js automatically.

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Port status notice

This PR is a core runtime fix (synthetic call_id format compliance with the ^[a-zA-Z0-9_-]+$ pattern enforced by realtime backends) and would normally be ported to agents-js. However, the AsyncToolset feature itself has not yet landed in agents-js main — it is currently being introduced via the open PR livekit/agents-js#1397 ("Add non-blocking tool calls").

Because there is no async_toolset.ts on agents-js main to patch, a standalone port PR cannot be opened today. Instead, the routine will leave a note on livekit/agents-js#1397 so the equivalent _ (underscore) call_id format is incorporated into that branch before it merges, avoiding the same realtime-backend rejection on the JS side.

cc @toubatbrian @livekit/agent-devs

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