fix: update AgentRuntime CRD to remove default 8h maxSessionDuration

Remove the 'default: 8h' value and drop maxSessionDuration from the
required list in the generated CRD YAML. This matches the updated
kubebuilder markers in agent_type.go where +kubebuilder:default was
removed and the field marked as +optional.

Fixes codegen check failure in PR volcano-sh#360.

Author: HarshitPal25

manifests/charts/base/crds/runtime.agentcube.volcano.sh_agentruntimes.yaml

@@ -44,10 +44,11 @@ spec:
             description: Spec defines the desired state of the AgentRuntime.
             properties:
               maxSessionDuration:
-                default: 8h
                 description: |-
                   MaxSessionDuration describes the maximum duration for a session.
                   After this duration, the session will be terminated no matter active or inactive.
+                  When omitted, sessions have no maximum duration and will only be cleaned up
+                  by idle timeout (SessionTimeout).
                 type: string
               podTemplate:
                 description: PodTemplate describes the template that will be used
@@ -8504,7 +8505,6 @@ spec:
                   type: object
                 type: array
             required:
-            - maxSessionDuration
             - podTemplate
             - sessionTimeout
             - targetPort