Fix stdin-stream cancel race and add integration test suite

[cte]

Summary

• fix stdin-stream request attribution and queue promotion handling for follow-up messages in apps/cli/src/commands/cli/stdin-stream.ts
• harden cancel behavior in src/core/webview/ClineProvider.ts so startup-time cancel does not fail when task history is not persisted yet
• fix attempt-completion follow-up routing in src/core/tools/AttemptCompletionTool.ts
• replace ad-hoc stdin repro scripts with a reusable integration runner in apps/cli/scripts/integration
• rename CLI integration command to pnpm test:integration

Integration coverage added

• cancel-active-task
• cancel-immediately-after-start-ack
• followup-after-completion
• followup-during-streaming
• message-without-active-task
• multi-message-queue-order
• shutdown-while-running
• start-while-busy

Validation

• npx pnpm --dir apps/cli build:extension
• npx pnpm --dir apps/cli exec tsc --noEmit --pretty false
• npx pnpm --dir apps/cli test:integration (8/8 passed)
• pre-push check-types also passed via repo hooks

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