Add typed JSON output to theme preview

[gonzaloriestra]

WHY are these changes introduced?

Preview JSON should expose a stable resource and wait for browser diagnostics.

WHAT is this pull request doing?

Return a strict preview resource with an opaque string ID and validated URL. Keep the native service compatibility adapter, await browser warnings before final output, and retain explicit environment batches.

The public output follows docs/cli/json-output.md; explicitly named environments use one {environments: [...]} object.

How to manually test your changes?

shopify theme preview --store example.myshopify.com --json
shopify theme preview --json-schema

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
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

[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/public/node/base-command.d.ts

@@ -35,6 +35,7 @@ declare abstract class BaseCommand extends Command {
         argv: string[];
     }>;
     protected environmentsFilename(): string | undefined;
+    protected validateNonTTYFlags(flags: FlagOutput): void;
     protected failMissingNonTTYFlags(flags: FlagOutput, requiredFlags: string[]): void;
     private failMissingNonTTYFlagRequirements;
     private applicableNonTTYFlagRequirements;

packages/cli-kit/dist/public/node/themes/theme-manager.d.ts

@@ -11,5 +11,12 @@ export declare abstract class ThemeManager {
     findOrCreate(name?: string, role?: Role): Promise<Theme>;
     fetch(name?: string, role?: Role): Promise<Theme | undefined>;
     generateThemeName(context: string): string;
-    create(themeRole?: Role, themeName?: string): Promise<Theme>;
+    create(themeRole?: Role, themeName?: string): Promise<{
+        createdAtRuntime: boolean;
+        id: number;
+        name: string;
+        processing: boolean;
+        role: string;
+        src?: string;
+    }>;
 }
\ No newline at end of file

[github-actions]

⚠️ Potential Breaking Changes Detected

This PR contains changes that may break the existing contract.

@shopify/dev_experience — this PR contains breaking changes that require coordination for the next major release.

📦 Major Version Changesets

The following changesets request a major version bump:

Changeset	Package
theme-duplicate-json-contract.md	'@shopify/cli': major
theme-info-json-contract.md	'@shopify/cli': major
theme-list-json-contract.md	'@shopify/cli': major
theme-open-json-contract.md	'@shopify/cli': major
theme-preview-json-contract.md	'@shopify/cli': major
theme-push-json-contract.md	"@shopify/cli": major