fix(desktop): omit undefined fields from renderer IPC payloads

[Hona]

Closes #50583, closes #50153, closes #50503, closes #50177, closes #50173, closes #50309

The composer's attach button, the folder picker, save dialogs and openPath all fail with Desktop IPC handler failed since 2.0.11. The native dialog never opens.

Why

#50112 made the renderer post RPC payloads over the MessagePort directly (structured clone, no RpcClient). The main process still decodes with Schema.toCodecJson, where Schema.optional(String) means key absent | string | null. Structured clone keeps a present-but-undefined key, so the request dies in schema decoding before the handler runs.

flowchart LR
  A["files.ts<br/>{ title: options?.title }<br/>→ title: undefined"] --> B["ipc-client.ts<br/>port.postMessage(raw)"]
  B --> C["ipc-transport.ts<br/>codecFor: toCodecJson"]
  C -- "Expected string | null<br/>at [&quot;options&quot;][&quot;title&quot;]" --> D["Cause.Die"]
  D --> E["toast: Desktop IPC handler failed<br/>(cause never shown)"]
  style D fill:#ff5c7a,color:#000

Loading

Before #50112, RpcClient encoded the payload through the same schema and dropped the undefined optionals on the way out. This restores that behaviour at the renderer boundary.

Change

• omitUndefined drops undefined properties from a request payload the way JSON.stringify would, leaving Uint8Array payloads untouched.
• ipc-client.ts runs every outgoing request payload through it.

// renderer → main, before
{ options: { multiple: true, title: undefined, defaultPath: "C:\\proj", extensions: undefined } }
// after
{ options: { multiple: true, defaultPath: "C:\\proj" } }

The transport test drives the real FilesOpenFilePicker schema through IpcServerProtocolLive with a raw renderer-style request and asserts both the rejection without normalisation and the success with it.

Affected requests

RPC	undefined field	Caller
FilesOpenFilePicker	title, extensions	attach button, Ctrl+U
FilesOpenDirectoryPicker	title	folder picker
FilesSaveFile	title	save/export
FilesOpenPath	application	open path without an app

Drag & drop and clipboard paste bypass these RPCs, which is why they kept working.

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