[Performance] Memoize isWsl check

[gonzaloriestra]

WHY are these changes introduced?

The isWsl check involves a dynamic import and internal logic that is constant for the duration of a CLI process. Repeatedly calling it adds unnecessary overhead in high-frequency paths.

WHAT is this pull request doing?

Memoizes the isWsl promise in packages/cli-kit/src/public/node/system.ts so subsequent calls return the same cached promise. Added unit tests to verify memoization and a reset function for testing isolation.

How to test your changes?

CI

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

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PR created automatically by Jules for task 3450321297428259022 started by @gonzaloriestra

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[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/system.d.ts

@@ -111,6 +111,10 @@ export declare function isCI(): boolean;
  * @returns True if the current environment is a WSL environment.
  */
 export declare function isWsl(): Promise<boolean>;
+/**
+ * Resets the memoized WSL check.
+ */
+export declare function _resetIsWsl(): void;
 /**
  * Check if stdin has piped data available.
  * This distinguishes between actual piped input (e.g., )