fix(validate-on-module-load): move validation timing to module load phase

[kim-sung-jee]

Resolved an issue where validation was executed during file import, causing unintended side effects. Validation now occurs when the module is loaded, ensuring proper lifecycle alignment.

---

Example Scenario

The following scenario describes the environment:
There are two entry modules: ApiModule and ConsumerModule.

@Module({
  imports: [
    ConfigModule.forRoot({
      validationSchema: Joi.object({ ...ApiEnvs }),
    }),
  ],
})
export class ApiModule {}

@Module({
  imports: [
    ConfigModule.forRoot({
      validationSchema: Joi.object({ ...ConsumerEnvs }),
    }),
  ],
})
export class ConsumerModule {}

In the project's root main.ts, these two modules are imported:

if (serverType === 'API') {
  await NestFactory.create(ApiModule);
} else if (serverType === 'CONSUMER') {
  await NestFactory.create(ConsumerModule);
} ...

In the current version, all validation functions are executed regardless of the serverType.

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): Tests were not added as they were considered unnecessary for this change.
• 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?

Issue Number: N/A

Validation functions in ConfigModule.forRoot are executed regardless of module loading.

What is the new behavior?

• Validation is now included in the factory function of the VALIDATED_ENV_LOADER provider. This ensures it runs during module initialization via NestJS DI and centralizes the responsibility of validating and loading environment variables.

Does this PR introduce a breaking change?

• Yes
• No

Other information