Add test cases for nested calls and applicable range

Added two new test cases:
1. TestSignatureHelpNestedCalls: Validates that outer calls take precedence when the cursor is outside inner call's arguments, as per issue #1419
2. TestSignatureHelpApplicableRange: Validates that signature help is not shown for positions outside the call

Both tests pass, confirming the fix works correctly.

Co-authored-by: jakebailey <[email protected]>

Author: Copilot

internal/fourslash/tests/signatureHelpApplicableRange_test.go

@@ -0,0 +1,26 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestSignatureHelpApplicableRange(t *testing.T) {
+	t.Parallel()
+
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	const content = `let obj = {
+    foo(s: string): string {
+        return s;
+    }
+};
+
+let s =/*a*/ obj.foo("Hello, world!")/*b*/  
+  /*c*/;`
+	f := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	
+	// Markers a, b, c should NOT show signature help (outside the call)
+	f.VerifyNoSignatureHelpForMarkers(t, "a", "b", "c")
+}

internal/fourslash/tests/signatureHelpNestedCalls_test.go

@@ -0,0 +1,39 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestSignatureHelpNestedCalls(t *testing.T) {
+	t.Parallel()
+
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	const content = `function foo(s: string) { return s; }
+function bar(s: string) { return s; }
+let s = foo(/*a*/ /*b*/bar/*c*/(/*d*/"hello"/*e*/)/*f*/);`
+	f := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	
+	// Markers a, b, c should show foo (outer call)
+	f.GoToMarker(t, "a")
+	f.VerifySignatureHelp(t, fourslash.VerifySignatureHelpOptions{Text: "foo(s: string): string"})
+	
+	f.GoToMarker(t, "b")
+	f.VerifySignatureHelp(t, fourslash.VerifySignatureHelpOptions{Text: "foo(s: string): string"})
+	
+	f.GoToMarker(t, "c")
+	f.VerifySignatureHelp(t, fourslash.VerifySignatureHelpOptions{Text: "foo(s: string): string"})
+	
+	// Marker d should show bar (inside inner call)
+	f.GoToMarker(t, "d")
+	f.VerifySignatureHelp(t, fourslash.VerifySignatureHelpOptions{Text: "bar(s: string): string"})
+	
+	// Markers e, f should show foo (after inner call)
+	f.GoToMarker(t, "e")
+	f.VerifySignatureHelp(t, fourslash.VerifySignatureHelpOptions{Text: "foo(s: string): string"})
+	
+	f.GoToMarker(t, "f")
+	f.VerifySignatureHelp(t, fourslash.VerifySignatureHelpOptions{Text: "foo(s: string): string"})
+}