Adopt existing AWS Mac fleets safely and add Yukon submission support

[morganmcg1]

Depends on

• Add Docker and AWS GPU launch backends #3472, rebased onto the current main before this stack was cut.

Summary

• Add schema-v1 manifests for adopting an exact, already-running AWS Mac fleet.
• Map one named student to each instance without creating or terminating adopted EC2 instances.
• Isolate each reused host under a tag-scoped runtime with per-launch ownership nonces.
• Add optional Yukon bundle/token installation while preserving the legacy MLXFast path.

Safety

• Keep preflight read-only while validating the full EC2 snapshot, Dedicated Host mapping, IMDS identity, prerequisites, prior unloaded launch daemons, and unused run namespaces.
• Ignore ambient SSH config, agents, and global host keys; require the imported key and pinned known_hosts file.
• Revalidate instance/network identity before every adopted launch or cleanup action.
• Require matching ownership tokens before remote deletion and checkpoint cleanup per node so partial four-host cleanup is retry-safe.
• Preserve adopted instances, Dedicated Hosts, networking, shared security groups, external SSH material, and prior run roots.

CLI and operations

• Reuse uses --aws_mac_nodes_path ... --aws_mac_bootstrap_mode reuse --aws_ttl_hours 0.
• --aws_mac_yukon_bundle selects Yukon and YUKON_API_TOKEN; an empty value retains MLXFast and MLXFAST_API_TOKEN.
• Mixed created/adopted fleets are intentionally rejected.

Validation

• Focused AWS Mac/native/launch suite: 110 passed, 25 subtests passed.
• Full suite: 1,266 passed, 6 skipped, 48 subtests passed.
• py_compile and git diff --check pass.
• The exact four-instance launch command passes --dry_run and read-only
  --preflight_only, including credentials, repository access, EC2 snapshots,
  strict SSH, IMDS identity, prerequisites, prior daemon state, and namespace
  collision checks.