refactor: eliminate circular getStorageService() calls in cleanup methods

[Copilot]

Addresses review feedback from PR #478 to improve code clarity in the storage service initialization flow.

Changes:

• Pass Storage as parameter to fixFolderConnectionStrings() and cleanupDuplicateConnectionStringParameters() instead of calling getStorageService() internally
• Obtain storage service once in resolvePostMigrationErrors() and pass to both cleanup methods
• Clarify test comment to specify that setTimeout waits for the fire-and-forget cleanupOrphanedItems(), not the awaited cleanup methods

Before:

private static async cleanupDuplicateConnectionStringParameters(context: IActionContext): Promise<void> {
    for (const connectionType of [ConnectionType.Clusters, ConnectionType.Emulators]) {
        const storageService = await this.getStorageService();  // Called during initialization - appears circular
        const items = await storageService.getItems<StoredItemProperties>(connectionType);
        // ...
    }
}

After:

private static async resolvePostMigrationErrors(): Promise<void> {
    const storageService = await this.getStorageService();  // Obtained once
    await this.fixFolderConnectionStrings(context, storageService);
    await this.cleanupDuplicateConnectionStringParameters(context, storageService);
}

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