fix(common): emit type errors for optional fields when WasValidated is true

[bas0N]

Closes #2182

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?

When using ConfigService<Config, true> with a configuration type that includes optional fields (e.g., from Zod's .optional()), TypeScript does not emit type errors when assigning string | undefined to a strict string type.

This happens because the R type parameter in the get/getOrThrow overloads is inferred from contextual typing (the expected return type) instead of using the default PathValue<T, P>.

Example:

type Config = { API_KEY: string | undefined };

class Service {
  private apiKey: string;
  constructor(private config: ConfigService<Config, true>) {
    // Should error but doesn't!
    this.apiKey = this.config.get('API_KEY', { infer: true });
  }
}

Issue Number: #2182

What is the new behavior?

TypeScript now correctly emits type errors when assigning potentially undefined values to strict types, regardless of the WasValidated parameter.

The fix removes the R type parameter from the affected overloads and uses PathValue<T, P> directly in the return type, preventing contextual typing from overriding the correct type inference.

Does this PR introduce a breaking change?

• Yes
• No

The fix is compile-time only and does not change runtime behavior. Code that previously compiled without errors but was incorrectly typed will now show appropriate type errors, which is the expected/correct behavior.

Other information

Affected method overloads:

• get(propertyPath, options)
• get(propertyPath, defaultValue, options)
• getOrThrow(propertyPath, options)
• getOrThrow(propertyPath, defaultValue, options)

Added test file tests/e2e/type-inference-validated.spec.ts that uses @ts-expect-error to verify correct type inference behavior for both WasValidated = true and WasValidated = false cases