[Feature]: Support cloud fleet in-place update for `idle_duration`

[r4victor]

Problem

Support in-place update for cloud fleets when idle_duration changes. This is useful for increasing the lifetime of already provisioned instances as well as increasing/decreasing the lifetime of future instances.

Solution

Today idle timeout is stored on each instance (termination_policy / termination_idle_time) and enforced from the instance row, so updating fleet idle_duration won't affect existing fleet instances automatically.

The intended behavior is:

• persist the updated fleet spec in place
• reconcile existing cloud fleet instances to the new idle policy
• apply the new idle policy to future fleet instances as well

This should be done in fleet reconciliation/background logic

A special case to define during implementation: instances provisioned via submitted jobs may have run-influenced idle_duration. The question is whether idle_duration should be update for such instances. I'm inclined to yes since users need to be able to prolong the lifetime of instances no matter how they've been provisioned.

Workaround

No response

Would you like to help us implement this feature by sending a PR?

Yes