Remove unused extension creation surface

[isaacroldan]

Summary:

• Remove the unused ensureThemeExtensionDevContext helper.
• Delete the now-unused extension creation helper, GraphQL mutation, and client method surface.
• Clean up tests and stubs that only covered the deleted path.

Validation:

• pnpm vitest run packages/app/src/cli/services/context.test.ts packages/app/src/cli/services/context/id-matching.test.ts packages/app/src/cli/services/context/id-manual-matching.test.ts packages/app/src/cli/services/context/identifiers-extensions.test.ts packages/app/src/cli/services/dev/create-extension.test.ts
• pnpm --filter app type-check
• pnpm --filter app lint
• pnpm knip

[isaacroldan]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Remove unused extension creation surface #8023 👈 (View in Graphite)
• Remove atomic deployment support flag #8022
• Remove store search support flag #8021
• Remove legacy developer preview concepts #8020
• Remove dev session support branching #8019
• Remove deprecated draftable extension process #8018
• Remove deprecated 1P dev mode #8017
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[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/constants.d.ts

@@ -6,7 +6,6 @@ export declare const environmentVariables: {
     doctor: string;
     enableCliRedirect: string;
     env: string;
-    firstPartyDev: string;
     noAnalytics: string;
     optOutInstrumentation: string;
     appAutomationToken: string;

packages/cli-kit/dist/public/node/context/local.d.ts

@@ -63,13 +63,6 @@ export declare function alwaysLogAnalytics(env?: NodeJS.ProcessEnv): boolean;
  * @returns True if SHOPIFY_CLI_ALWAYS_LOG_METRICS is truthy.
  */
 export declare function alwaysLogMetrics(env?: NodeJS.ProcessEnv): boolean;
-/**
- * Returns true if the CLI User is 1P.
- *
- * @param env - The environment variables from the environment of the current process.
- * @returns True if SHOPIFY_CLI_1P is truthy.
- */
-export declare function firstPartyDev(env?: NodeJS.ProcessEnv): boolean;
 /**
  * Returns true if the CLI can run the "doctor-release" command.
  *