fix: task submission context cancellation, executions success-rate calc

submitTask was using the inbound HTTP/MCP request context to submit work to
the executor pool, so every chatbot/REST-submitted task was cancelled the
moment the request returned rather than actually running. Use the server's
long-lived lifecycle context instead, matching the other Submit call sites.

Separately, computeExecutionStats compared execution state against a
lowercase 'completed' literal while the backend always emits uppercase
state values, so success rate silently read 0% for every execution. Also
treat READY as success alongside COMPLETED, since a top-level task's
execution lands at READY on success — COMPLETED requires a later, separate
accept step.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VTUSAEKfsPc6WGDq45yPHD

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