Introduce a structured NodeConfig that replaces the flat
EphemeralNodeInactivityTimeout field with a nested Node section.
Add node.expiry config (default: no expiry) as the unified default key
expiry for all non-tagged nodes regardless of registration method.
Remove oidc.expiry entirely — node.expiry now applies to OIDC nodes
the same as all other registration methods. Using oidc.expiry in the
config is a hard error. determineNodeExpiry() returns nil (no expiry)
unless use_expiry_from_token is enabled, letting state.go apply the
node.expiry default uniformly.
The old ephemeral_node_inactivity_timeout key is preserved for
backwards compatibility.
Updates #1711
4d0b273b90