Add optional websocket keepalive pings for idle Exec sessions

[Copilot]

Exec websocket sessions can be dropped around the 5-minute mark in some proxy/network paths when no traffic is sent. This change adds an opt-in keepalive mechanism for Exec without changing default behavior.

• API: opt-in exec keepalive

  • Added ExecOptions with pingIntervalMs?: number.
  • Extended Exec.exec(...) with an optional trailing options parameter.

• Behavior: periodic websocket ping

  • When pingIntervalMs is a positive integer, Exec sends websocket ping frames on that interval while the socket is open.
  • Keepalive is ignored when the underlying socket does not expose ping().

• Lifecycle: cleanup on termination

  • Keepalive timer is cleared on websocket close and error to avoid orphaned intervals.

• Coverage: focused unit test

  • Added a targeted exec_test case validating that pings are emitted when configured and stop after close.

const exec = new Exec(kc);

await exec.exec(
  'ns',
  'pod',
  'container',
  ['sh', '-c', 'long-running-command'],
  stdout,
  stderr,
  stdin,
  false,
  undefined,
  { pingIntervalMs: 30_000 },
);

[k8s-ci-robot]

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[mstruebing]

/lgtm

Will leave it in this state for now. to give others the chance to look at it.