Fix panic in checker.Type.Types for non-union types via the API (#4426)

[UditDewan]

Fixes #4426.

Problem — The public getter getTypes() (Type.getTypes() → Session.handleGetTypesOfType → checker.Type.Types) forwards an arbitrary
user type to Type.Types, an invariant accessor that only handles union/intersection and template-literal types and panic("Unhandled
case in Type.Types")s on everything else. Calling getTypes() on e.g. the intrinsic string crashes the request. Unlike the
getTypeParameters/getCheckType family (defended JS-side by ?? [] defaults and handle gates), getTypes() always reaches the server,
so the panic is reachable directly through type.getTypes(). Same class as #4338 (getTypeArguments) and #4335/#4425 (getBaseTypes).

Fix — Guard at the API boundary (internal/api/session.go); non-union types return []. The Type.Types accessor stays strict (meant
to panic on misuse).

Tests — New Type - getTypes block (sync + async): union returns 2 constituents; non-union returns [] (regression for #4426).

Test plan — go build ✓, gofmt clean ✓, 4/4 new tests pass (non-union panics on main, passes here) ✓.

Claude (Opus 4.8) helped investigate, reproduce, and test this fix.

[Copilot]

Pull request overview

This PR prevents a server-side panic when the public native-preview API Type.getTypes() is called on non-union/non-intersection/non-template-literal types (e.g. string), by adding an explicit guard at the API boundary in internal/api/session.go. It keeps checker.Type.Types() strict (panic on misuse) while making the public API resilient.

Changes:

• Add a flag-based guard in Session.handleGetTypesOfType so unsupported types return no constituents instead of panicking.
• Add sync native-preview tests verifying getTypes() returns two constituents for a union type.
• Add sync + async native-preview regression tests verifying getTypes() returns [] (and does not panic) for a non-union type.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/api/session.go	Guard getTypesOfType so only union/intersection/template-literal types call Type.Types(); otherwise return an empty result.
_packages/native-preview/test/sync/api.test.ts	Add sync regression coverage for Type.getTypes() on union and non-union types.
_packages/native-preview/test/async/api.test.ts	Add async regression coverage for Type.getTypes() on union and non-union types.