Bug: Custom agents via task tool broken with Anthropic BYOK provider

[knifeyspoony]

Summary

When using the Copilot SDK with the Anthropic BYOK provider (sweagent-anthropic), invoking a custom agent via the Task tool crashes with:

Error: message on turn 0 event missing callId

Custom agents work correctly with the Copilot API provider (sweagent-capi).

Root Cause

The Anthropic provider's agentic loop calls tool callbacks with only the input argument:

// Anthropic provider (sweagent-anthropic) — tool_use handling:
const result = await toolCallback(toolUseBlock.input);

The Copilot API provider passes a second context argument containing toolCallId:

// Copilot API provider (sweagent-capi) — tool execution:
const result = await toolCallback(input, { toolCallId: "..." });

The custom agent tool implementation reads toolCallId from this second argument to set the subagent's agentId. That agentId becomes the callId on the first progress event (turn 0, kind: "message"). When toolCallId is undefined (Anthropic path), agentId is undefined, and the SDK's own validation throws.

Reproduction

1. Configure a session with the Anthropic provider (BYOK API key, model e.g. claude-sonnet-4-5)
2. Have the agent invoke a subagent via the Task tool
3. Error: message on turn 0 event missing callId

Confirmed still broken on latest.