[api] Add .getNonMissingTypeOfSymbol() getter

[mrazauskas]

Close #4081

This PR adds the .getNonMissingTypeOfSymbol() getter.

[Copilot]

Pull request overview

Adds getNonMissingTypeOfSymbol to expose symbol types that respect exactOptionalPropertyTypes, addressing #4081.

Changes:

• Exposes the checker operation through the Go API and protocol.
• Adds synchronous and asynchronous TypeScript APIs.
• Tests behavior with exact optional property types enabled and disabled.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
internal/checker/exports.go	Exposes the checker method.
internal/api/session.go	Handles the new API request.
internal/api/proto.go	Registers the protocol method and parameters.
_packages/native-preview/src/api/sync/api.ts	Adds the synchronous getter.
_packages/native-preview/src/api/async/api.ts	Adds the asynchronous getter.
_packages/native-preview/test/sync/api.test.ts	Tests synchronous behavior.
_packages/native-preview/test/async/api.test.ts	Tests asynchronous behavior.

[mrazauskas]

By the way, currently in api/session handleGetTypeOfSymbol uses checker.GetTypeOfSymbol:

typescript-go/internal/api/session.go

Lines 1397 to 1410 in 8d29e62

	func (s *Session) handleGetTypeOfSymbol(ctx context.Context, params *GetTypeOfSymbolParams) (*TypeResponse, error) {
	setup, err := s.setupChecker(ctx, params.Snapshot, params.Project)
	if err != nil {
	return nil, err
	}
	defer setup.done()
	symbol, err := setup.resolveSymbolHandle(params.Symbol)
	if err != nil {
	return nil, err
	}
	return setup.newTypeResponse(setup.checker.GetTypeOfSymbol(symbol)), nil
	}

But in ls/api GetTypeOfSymbol uses checker.GetTypeOfSymbolAtLocation(symbol, nil)

typescript-go/internal/ls/api.go

Lines 39 to 44 in 8d29e62

	func (l *LanguageService) GetTypeOfSymbol(ctx context.Context, symbol *ast.Symbol) *checker.Type {
	program := l.GetProgram()
	checker, done := program.GetTypeChecker(ctx)
	defer done()
	return checker.GetTypeOfSymbolAtLocation(symbol, nil)
	}

Can this be an oversight? Because GetTypeOfSymbolAtLocation(symbol, nil) returns getNonMissingTypeOfSymbol() (exactly what I try to achieve, but adding a method to the API):

https://github.com/microsoft/typescript-go/blob/8d29e62f3585c2fb5179e4412895aa3a8f40b9f5/internal/checker/checker.go#L16433-L16470