feat(execution): workflow tool + MCP bridge run in-process

workflow_executor (workflow-as-agent-tool) short-circuits in executeTool
through WorkflowBlockHandler — the same invocation boundary canvas child
workflows use — mirroring the deployed_block_executor precedent. The
MCP serve bridge calls executeWorkflowService directly instead of
fetching its own execute endpoint; deployment-version pinning, MCP
response-size rejection, and the actor override become typed options
instead of header sniffing. Both callers drop the double admission slot
and duplicate top-level log row the HTTP hop cost, and failed child
runs now surface the structured error + child executionId so parents
and MCP clients can route on error class and hand providers a
reproducible handle.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CiHhAk2R1NryaS3R8n2yFz

Author: TheodoreSpeaks