fix(vmm): validate an update before writing any of it

[kvinwang]

Problem

update_vm writes the compose file, encrypted env and user config, grows the data disk with qemu-img resize, and releases the VM's network interfaces before running the checks that can still reject the request (GPU, port mapping, networking, NIC agreement, key provider). A rejected update therefore leaves a new compose hash, env and grown disk against the old manifest.

For example, an update that adds a port the node's cvm.port_mapping refuses (#1286) still replaces the stored compose file:

assertion `left == right` failed
  left: "{\"manifest_version\":\"2\",\"name\":\"new\",\"runner\":\"docker-compose\"}"
 right: "{}"

Fix

Build and validate the new manifest in memory first, then apply side effects in one place: grow the disk, release interfaces, write compose/env/user config, write the manifest. Disk growth moves out of apply_resource_updates into grow_disk_if_needed, which resize_vm also calls. The swtpm decision reads the requested compose instead of the one just written; the result is the same.

Stacked on #1286.

Verification

New a_rejected_update_writes_nothing fails on #1286 (output above) and passes here. cargo test -p dstack-vmm (212 passed), cargo clippy -p dstack-vmm --bins -- -D warnings, cargo fmt --check.