feat(sandbox): keep mount endpoints stable across sandbox restarts

[NathanFlurry]

Problem

The native sandbox_agent mount currently receives a concrete Sandbox Agent baseUrl when the VM is created. That endpoint is not stable across sandbox replacement: Docker allocates a new host port, and hosted providers may return a different URL after recreation or resume. A lazy sandbox that shuts down while the VM remains alive therefore cannot safely reuse the original mount descriptor.

Desired behavior

• Give each AgentOS VM a stable sandbox endpoint for the lifetime of the VM.
• Route that endpoint through a host relay, mutable resolver, or equivalent indirection to the currently active Sandbox Agent client.
• Let both the native filesystem mount and process bindings share the same current client and lazy lifecycle.
• Update authentication headers/tokens when the backing sandbox changes without remounting or rebooting the VM.
• Fail clearly while startup is in progress or has failed; concurrent calls must remain single-flight.
• Add native coverage that accesses the same mounted path before and after replacing the backing sandbox without restarting the AgentOS VM.

Provider sandbox IDs may remain stable for reconnect-capable backends, but their network address must not be treated as a cross-provider invariant.