Add cancel control for task polling

[HarperZ9]

Summary

Adds a cancel control for tool calls that are running as MCP tasks and currently being polled from the Tools tab.

Root cause

The app already exposed tasks/cancel through the Tasks tab, but a user who launched a tool as a task from the Tools tab only saw the disabled Polling Task... button. The active polling task ID was not stored in a way that Tools could cancel, and the polling loop could continue into tasks/get after a user wanted to stop it.

Changes

• Track the active polling task ID/run in App.
• Pass pollingTaskId and cancelTaskPolling into ToolsTab.
• Render a scoped Cancel Task button while a tool task is actively polling.
• Wake and stop the local polling delay after successful cancellation.
• Guard against late tasks/get / tasks/result responses overwriting the cancelled state.
• Add regression coverage for direct cancellation and an in-flight polling response race.

Closes #1039.

Validation

• npm.cmd test -- --runTestsByPath src/__tests__/App.taskPolling.test.tsx --runInBand
• npm.cmd run build-client
• npm.cmd run lint
• npx.cmd prettier --check client/src/App.tsx client/src/components/ToolsTab.tsx client/src/__tests__/App.taskPolling.test.tsx
• git diff --check