Expose createInteractiveAiSession on route/loader PluginContext alongside the
one-shot createAiSession. The prompt->parse->retry->pause->resume loop is
reimplemented in an engine seam modeled on planning.ts (no engine await-input
primitive exists). Reuses PlanningQuestion/PlanningResponse; stays generic with
no plugin-specific concepts. Injected on route contexts only; tool/runtime
contexts omit it (parity with createAiSession).
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