Fix for issue 1750-prevent re-execution of already running cell

[SsaiSanjanna03]

trafficstars

Fixes https://github.com/jupyter/notebook/issues/1750:Prevent concurrent cell executions in Jupyter Notebook.

Changes:

• Updated packages/notebook/src/actions.tsx to use metadata['running'] to block execution if a cell is running, logging "Cell is already running".
• Added UI test in ui-tests/tests/runcell.spec.ts to verify:
• • Single output for rapid cell executions.
• • Correct variable state (i == 1).
• • Console log for ignored requests.

Testing:

• Manually verified by rapid Shift+Enter on a long-running cell.

Related:

• Closes https://github.com/jupyter/notebook/issues/1750

Ready for review.