Idle-shutdown feature follow-up items

[aws-prayags]

1/ Support for partial override - such as only specifying timeout and not the detection method
2/ Add spec validation for:
a/ at least one detection method must be specified if idleShutdown is enabled
b/ for MinTimeoutMinutes / MaxTimeoutMinutes values'
3/ Update resolved spec ( template + workspace spec ) in workspace status
4/ Failure case handling - unable to hit specified endpoint
5/ Address comment - https://github.com/jupyter-infra/jupyter-k8s/pull/88/files#r2470862253
6/ Add unit tests for PR - https://github.com/jupyter-infra/jupyter-k8s/pull/88/files