refactor(schematics): use explicit jest imports in templates

[Generalsimus]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Currently, when generating components using the Nest CLI (e.g., nest g resource), the generated .spec.ts files rely on implicit Jest global variables (describe, beforeEach, it, expect). The default package.json schematic also installs @types/jest to support this. This often results in type errors in test files right out of the box in strictly configured environments, as the dependencies are not explicitly declared.

Issue Number: N/A

What is the new behavior?

The templates have been refactored to prioritize explicit dependencies:

1. The package.json schematic now installs @jest/types instead of @types/jest.
2. All generated .spec.ts files now include explicit imports at the top of the file (e.g., import { describe, beforeEach, it, expect } from '@jest/globals';).
   This eliminates type errors in newly generated components and aligns the boilerplate with strict clean-code practices by avoiding global namespace pollution.

Does this PR introduce a breaking change?

• Yes
• No

Other information

This change only affects newly scaffolded files and projects. Existing tests using implicit globals will continue to work normally.

[kamilmysliwiec]

We're about to migrate to vitest in the next release #2302