types: correct Model.schema type and fix unknown check for this param type in schema.methods

[vkarpov15]

Summary

Fix #15693

Model.schema currently is a generic schema rather than TSchema, and ThisParameter<T[K], unknown> extends unknown is trivially true because everything extends unknown.

Examples

[Copilot]

Pull Request Overview

This PR fixes two type issues in Mongoose:

1. Corrects the Model.schema property type to use the more specific TSchema generic parameter instead of Schema<TRawDocType>
2. Fixes a bug in the AddThisParameter utility type where ThisParameter<T[K], unknown> extends unknown was always true, preventing proper detection of explicit this parameters in schema methods

Reviewed Changes

Copilot reviewed 1 out of 3 changed files in this pull request and generated 1 comment.

File	Description
types/utility.d.ts	Replaces IfUnknown helper with IsUnknown that returns boolean literal types, and fixes the AddThisParameter type to properly check if a function already has an explicit this parameter before injecting one
types/models.d.ts	Changes Model.schema property type from Schema<TRawDocType> to TSchema to preserve the full schema type information
test/types/models.test.ts	Adds test case demonstrating that methods with explicit this parameters preserve their type while methods without get the hydrated document type injected

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