Clarify enabled/admin-status descriptions for coherent transceiver low-power-mode

[earies]

• (M) release/models/interfaces/openconfig-interfaces.yang
  • Update enabled leaf description to indicate that disabling
    an interface SHOULD place a pluggable coherent transceiver
    (e.g. ZR/ZR+) into low-power-mode, including channelized
    cases where all channels sharing a module must be disabled
  • Mirror clarification on admin-status leaf and DOWN enum
  • Scope behavior to coherent optics; gray (direct-detect) and
    non-pluggable interfaces are unaffected
  • Increment version to 3.9.0

Change Scope

This PR amends descriptions only. No structural or type changes are made
to the model.

The enabled configuration leaf and admin-status operational state leaf
(including the DOWN enum) are clarified to indicate that, on interfaces
backed by a pluggable coherent transceiver (e.g. ZR/ZR+), administratively
disabling the interface SHOULD additionally place the transceiver into
low-power-mode. When a coherent module is shared by multiple channelized
interfaces, low-power-mode is only entered once all channels sharing the
module have been disabled.

This behavior is explicitly scoped to coherent optics. Gray
(direct-detect) optics and non-pluggable interfaces are not in scope, even
where the underlying module would otherwise support a low-power mode.

While these are description-only changes, behavior is being clarified for
a pre-existing leaf and may require implementations to adjust. The model
is therefore bumped as a minor version 3.8.1 -> 3.9.0. If folks feel
otherwise, this could be treated as a patch update - e.g. 3.8.2

Platform Implementations

N/A

Tree View

No tree changes - only description updates

[gemini-code-assist]

Code Review

This pull request updates the openconfig-interfaces YANG module to version 3.9.0, clarifying that coherent transceivers should enter low-power-mode when an interface is administratively disabled or in a DOWN state, provided all shared channels are also disabled. A review comment suggests enhancing the admin-status description to explicitly include the multi-channel condition for better consistency with other updated fields.

[earies]

@dplore @ElodinLaarz for review/comment