Keep JSON prompts and concurrent process output on stderr

[isaacroldan]

WHY are these changes introduced?

Interactive prompts and concurrent process output can write terminal text to stdout in JSON mode, which prevents commands from returning one parseable JSON document.

This is the shared UI prerequisite for #8801.

WHAT is this pull request doing?

Send finite concurrent process output as JSON diagnostic and progress events to stderr. Send interactive UI to stderr in JSON mode while preserving input policy and streaming behavior.

How to manually test your changes?

With dependent PR #8801 checked out, use a disposable local app with installed dependencies:

1. Run shopify app build --path <app> --skip-dependencies-installation --json --no-input > build.json 2> build.events.jsonl. Check that stdout contains one result and build output appears as events on stderr.
2. After deleting a previously selected config in an app with at least two remaining configs, repeat without --no-input. Select a config and check that the prompt appears on stderr while stdout remains valid JSON.
3. Repeat without --json and check that normal build output remains.

Validation

70 focused UI tests passed, including existing task and concurrent-output tests. CLI Kit build, type checks, scoped lint, and Knip passed. Full CI and other operating systems were not run locally.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• Added a CLI Kit patch changeset.

[github-actions]

Differences in type declarations

We detected differences in the type declarations generated by Typescript for this branch compared to the baseline ('main' branch). Please, review them to ensure they are backward-compatible. Here are some important things to keep in mind:

• Some seemingly private modules might be re-exported through public modules.
• If the branch is behind main you might see odd diffs, rebase main into this branch.

New type declarations

We found no new type declarations in this PR

Existing type declarations

packages/cli-kit/dist/private/node/ui/components/ConcurrentOutput.d.ts

@@ -1,4 +1,4 @@
-import { OutputProcess } from '../../../../public/node/output.js';
+import { type OutputProcess } from '../../../../public/node/output.js';
 import { AbortSignal } from '../../../../public/node/abort.js';
 import { FunctionComponent } from 'react';
 export interface ConcurrentOutputProps {
@@ -14,6 +14,8 @@ interface ConcurrentOutputContext {
     stripAnsi?: boolean;
 }
 declare function useConcurrentOutputContext<T>(context: ConcurrentOutputContext, callback: () => T): T;
+/** Runs finite processes concurrently and routes their output through the shared diagnostic context. */
+export declare function runConcurrentProcessesForJson({ processes, abortSignal, }: Pick<ConcurrentOutputProps, 'processes' | 'abortSignal'>): Promise<void>;
 /**
  * Renders output from concurrent processes to the terminal.
  * Output will be divided in a three column layout