Add testcases for Partial<T> rest spreading with keyof T

[Copilot]

This PR adds comprehensive test coverage for the prior commit that changed rest spreading behavior to use Partial<T> instead of Omit<T, keyof T> when the omit key contains keyof T.

Background

The previous commit implemented a suggestion from microsoft#62572 to improve the type inference for rest spreading when using keyof T as the omitted key. Previously, the type system would produce Omit<T, keyof T>, but the more accurate type is Partial<T> since any property could potentially be omitted.

Test Coverage Added

This PR adds two complementary test files:

restSpreadingWithKeyofT.ts - Testing Partial behavior

Validates that Partial<T> is correctly used in 8 scenarios:

// Example: Using keyof T directly
function f1<T>(obj: T, key: keyof T) {
    const { [key]: removed, ...rest } = obj;
    return rest; // Type is Partial<T>
}

// Example: Union of keyof T parameters
function f2<T>(obj: T, k1: keyof T, k2: keyof T) {
    const { [k1]: _, [k2]: __, ...rest } = obj;
    return rest; // Type is Partial<T>
}

restSpreadingWithOmit.ts - Contrast cases

Shows when Omit<T, K> is still correctly used (6 test cases):

// Example: Specific literal key
function f1<T extends { a: string; b: number }>(obj: T) {
    const { a, ...rest } = obj;
    return rest; // Type is Omit<T, "a">
}

// Example: K extends keyof T (specific subset)
function f4<T, K extends keyof T>(obj: T, key: K) {
    const { [key]: removed, ...rest } = obj;
    return rest; // Type is Omit<T, K>, not Partial<T>
}

Validation

• ✅ All new tests pass with expected baselines
• ✅ Full test suite passes (98,897 tests)
• ✅ Lint and format checks pass
• ✅ Code review found no issues

These tests ensure the feature works correctly and provide clear documentation of the behavior for future maintainers.

Original prompt

Add testcases for the prior commit

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