Fix container machine setup script being dropped from artifact.

[anxkhn]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

When a container machine image ships a custom setup script as an OCI artifact,
that script was silently dropped and the default create-user.sh was used
instead.

In MachineClient.fetchMachineArtifact, the config layer is decoded into
resources: MachineResources? and the separate setup-script layer into
setupScript: String?. The two were merged with:

if var resources, let setupScript {
    resources.setupScript = setupScript
}

MachineResources is a value type (a struct), so if var resources binds a
mutable copy. Assigning setupScript mutates that throwaway copy, which is
discarded at the end of the block. The function then returns the outer
resources, whose setupScript is still nil. Downstream,
MachineBundle.create sees resources?.setupScript == nil and falls back to
copying the default userSetupFile, so the per-image script fetched from the
registry is never applied.

The fix merges through the optional (resources?.setupScript = setupScript) so
the value survives on the returned MachineResources. The merge is extracted
into a small merge(resources:setupScript:) helper, both to make the value
semantics explicit and to give the logic a unit-testable seam
(fetchMachineArtifact itself needs a live registry client).

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

Added a MachineAPIClientTests target with three tests for the merge:

• a non-nil setup script is applied to the returned resources,
• a nil setup script leaves an existing script untouched,
• a nil resources stays nil.

swift test --filter MachineAPIClientTests passes (3/3). Reverting the helper to
the original if var resources body makes the first test fail
(merged?.setupScript is nil), confirming the test covers this bug. The full
tree builds with swift build --build-tests, and swift format lint --strict
is clean on the changed files.

---

Files changed (3 files, +56 -3)

• Sources/Services/MachineAPIService/Client/MachineClient.swift
  Replace the inline if var resources, let setupScript { ... } merge with a call
  to a new static func merge(resources:setupScript:) -> MachineResources? that
  assigns through the optional. Doc comment notes the value-type pitfall.
• Package.swift
  Add a MachineAPIClientTests test target depending on MachineAPIClient
  (this target had no unit tests before, only VM-dependent integration tests).
• Tests/MachineAPIClientTests/MachineClientMergeTests.swift (new)
  Three Swift Testing (import Testing) regression tests. Carries the repo's
  canonical Apache-2.0 license header (byte-identical to existing tracked files).