Multiple image inputs collapse to "last image wins" in submitJob

[mattmillerai]

Surfaced by CodeRabbit on #10 (pre-existing logic; tracked here, not fixed in that comment-only PR).

`Transport.submitJob` loops over `request.inputs` and, for each `.image`, calls `patchLoadImageNodes(...)` which rewrites every `LoadImage` node in the workflow JSON. With more than one image input, each iteration overwrites the previous binding, so all `LoadImage` nodes end up pointing at the last uploaded filename.

Options:

1. Keyed mapping — associate each image input (by index/identifier) with a specific `LoadImage` node and patch only that node.
2. Interim guard — reject >1 image input with a clear `ComfyError` until keyed mapping exists:

```swift
if request.inputs.filter({ if case .image = $0 { return true }; return false }).count > 1 {
throw ComfyError.serverRejected(reason: .other("Multiple image inputs are not yet supported in a single request"))
}
```

Acceptance: a two-image request either binds each to its intended node, or fails fast with a descriptive error (no silent last-wins). Add a unit test covering the multi-image case.

[mattmillerai]

Interim guard merged in #22: submitJob now fails fast with ComfyError.serverRejected(.other("multiple_image_inputs_unsupported")) on >1 image input, so the silent "last image wins" behaviour is gone.

Remaining scope for this issue (kept open): keyed input→node mapping — associate each image input with a specific LoadImage node (by index/identifier) and patch only that node, so multi-image requests bind correctly instead of being rejected. Good candidate for 0.2.0.