Fix type parameter leak when using 'this' in reverse mapped types

[thromel]

Fixes #62779

Problem

When object literal methods reference this inside a function with a reverse mapped type parameter, the type parameter T was leaking through:

declare function test<T extends Record<string, unknown>>(obj: {
    [K in keyof T]: () => T[K];
}): T;

const obj = test({
    a() { return 0; },
    b() { return this.a(); },
});
// Expected: { a: number; b: number; }
// Actual:   { a: number; b: T[string]; } (widened to unknown)

Cause

In getContextualThisParameterType, when computing the this type for method b, the code was using the contextual type (the mapped type { [K in keyof T]: () => T[K] }) with unresolved type parameters. This caused this.a() to be typed as T["a"] rather than the already-inferred number.

Solution

When in an inference context, use the actual object literal type via checkExpressionCached(containingLiteral) rather than the contextual mapped type. This allows methods to see each other's already-inferred types when resolving this references.

Test Plan

• Added test case reverseMappedThisTypeInference.ts demonstrating the fix
• Verified all existing contextual, inference, and reverse mapped type tests pass