Add a CLI command to cancel an active kernel session

## Workflow

Automated GPU jobs need a way to stop a stale or runaway kernel session without deleting the kernel or using the web UI. The public SDK already exposes `ApiCancelKernelSessionRequest` at `POST /api/v1/kernels/cancel-session/{kernel_session_id}`, but the official CLI has no corresponding `kaggle kernels` command.

`kaggle kernels push` already prints the numeric `versionNumber`; that value is accepted by the cancellation endpoint as the kernel session id.

## Expected CLI

```bash
kaggle kernels cancel
```

The command should issue the existing SDK cancellation request and report that cancellation was requested. It should reject non-positive IDs and surface a backend rejection.

## Proposed scope

A small wrapper in `KaggleApi`, an argparse subcommand, focused unit coverage, and one documentation section. No change to the backend API or existing CLI behavior.

I have a tested implementation ready and can open a PR.

Start with the KaggleApi wrapper and the argparse entry point for `kaggle kernels push`, then trace the existing `ApiCancelKernelSessionRequest` at its cancellation endpoint. Done means `kaggle kernels cancel ` validates positive IDs, reports the requested cancellation, surfaces backend rejection, has focused unit coverage, and includes the planned documentation section.

Written by the indexing model from the issue text.