Fix isolatedDeclarations errors for arrow functions with default parameters

[Copilot]

isolatedDeclarations incorrectly reported TS9025/TS9013 errors on arrow function parameters that have both type annotations and default values when followed by required parameters:

// Both erroneously reported errors with isolatedDeclarations
export const foo = (one: string = "DEFAULT", two: string): string => { ... };        // TS9025
export const bar = (one: string | undefined = "DEFAULT", two: string): string => { ... }; // TS9013

Root cause: the pseudochecker's pseudo type for these parameters didn't match the checker's type during the equivalence check in serializeTypeForDeclaration, causing spurious ReportInferenceFallback calls.

Three fixes in internal/pseudochecker/lookup.go:

• cloneParameters: Was unconditionally setting Optional=true for parameters with initializers. Now matches the checker's isOptionalParameter semantics — a parameter with an initializer is optional only when all subsequent parameters are also optional/initialized/rest.
• typeFromParameter: When a typed parameter has an initializer and is followed by required parameters, the pseudo type now includes | undefined to match getTypeOfParameter.
• typeNodeCouldReferToUndefined: KindUndefinedKeyword was falling through to the default return false case (flagged by an existing TODO comment), causing string | undefined annotations to not be recognized as already containing undefined.

Also improves convergence with tsc on isolatedDeclarationsAddUndefined (2 errors → 1, matching tsc's count).