Add regression tests for #10043: backtick, @, and ! in type annotations (#19476)

Verifies that type annotations with invalid characters produce clear error messages:
- backtick: FS3563 'This is not a valid identifier'
- at sign: FS0615 'Unexpected infix operator in type expression'
- bang: FS1141 'Identifiers followed by ! are reserved for future use'

Fixes #10043

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: T-Gro

tests/FSharp.Compiler.ComponentTests/ErrorMessages/DiagnosticRegressionTests.fs

@@ -50,6 +50,43 @@ type Vehicle() = class end
         [ (Error 39, Line 3, Col 14, Line 3, Col 28, "The type 'OutOfScopeType' is not defined.")
           (Error 267, Line 3, Col 7, Line 3, Col 29, "This is not a valid constant expression or custom attribute value") ]
 
+// https://github.com/dotnet/fsharp/issues/10043
+[<Fact>]
+let ``Issue 10043 - backtick in type annotation should not report unexpected keyword`` () =
+    FSharp
+        """
+let i:float`1 = 3.0
+        """
+    |> typecheck
+    |> shouldFail
+    |> withDiagnostics
+        [ (Error 3563, Line 2, Col 12, Line 2, Col 13, "This is not a valid identifier")
+          (Error 10, Line 2, Col 13, Line 2, Col 14, "Unexpected integer literal in binding. Expected '=' or other token.") ]
+
+// https://github.com/dotnet/fsharp/issues/10043
+[<Fact>]
+let ``Issue 10043 - at sign in type annotation should report infix operator`` () =
+    FSharp
+        """
+let i:float@1 = 3.0
+        """
+    |> typecheck
+    |> shouldFail
+    |> withDiagnostics
+        [ (Error 615, Line 2, Col 12, Line 2, Col 13, "Unexpected infix operator in type expression") ]
+
+// https://github.com/dotnet/fsharp/issues/10043
+[<Fact>]
+let ``Issue 10043 - bang in type annotation should report reserved identifier`` () =
+    FSharp
+        """
+let i:float!1 = 3.0
+        """
+    |> typecheck
+    |> shouldFail
+    |> withDiagnostics
+        [ (Error 1141, Line 2, Col 7, Line 2, Col 13, "Identifiers followed by '!' are reserved for future use")
+          (Error 10, Line 2, Col 13, Line 2, Col 14, "Unexpected integer literal in binding. Expected '=' or other token.") ]
 
 // https://github.com/dotnet/fsharp/issues/7177
 [<Fact>]