actorTemplate(fix): update volume mount validation

[shrutiyam-glitch]

Fixes #1435 #1434

Two ActorTemplate validation gaps that pass creation today and fail at runtime, both closed with customValidation hooks (neither rule is expressible in tags):

• Fixes Container volume_mounts can reference undefined template volumes #1434 — a container volume_mount naming a volume the template never declares was accepted, leaving the golden actor permanently stuck in ACTOR_STATE_RESUMING. The rule spans two subtrees (containers and volumes), so the hook lives on CreateActorTemplateRequest.actor_template, where it sees the whole template.

• Fixes Duplicate mount_path for distinct volumes accepted within a container #1435 — two different volumes mounted at the same mount_path were accepted (the list-map key is name, so path collisions slipped through); The hook on Container.volume_mounts enforces path uniqueness within each container. This resolves the todo partially, and also discussed here - Declarative validation for ActorTemplate #1303 (comment)

• Tests pass

• Appropriate changes to documentation are included in the PR

[Michelle Au (msau42)]

Can you also test a nested path more than a level deep? Like /data and /data/a/b/c

[shrutiyam-glitch]

Done in 7085bfb