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

Tests that invalid characters in type annotation positions produce clear
error messages instead of confusing 'Unexpected reserved keyword':
- backtick: FS3563 'This is not a valid identifier'
- @: FS0615 'Unexpected infix operator in type expression'

Also removes deleted flaterrors.fs reference from fsproj.

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

Author: T-Gro

tests/FSharp.Compiler.ComponentTests/CompilerOptions/fsc/flaterrors.fs

@@ -0,0 +1,42 @@
+module ErrorMessages.DiagnosticRegressionTests
+
+open Xunit
+open FSharp.Test.Compiler
+
+// 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.") ]

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

@@ -300,6 +300,7 @@
     <Compile Include="ErrorMessages\MissingElseBranch.fs" />
     <Compile Include="ErrorMessages\MissingExpressionTests.fs" />
     <Compile Include="ErrorMessages\ModuleTests.fs" />
+    <Compile Include="ErrorMessages\DiagnosticRegressionTests.fs" />
     <Compile Include="ErrorMessages\NameResolutionTests.fs" />
     <Compile Include="ErrorMessages\SuggestionsTests.fs" />
     <Compile Include="ErrorMessages\TypeMismatchTests.fs" />
@@ -395,7 +396,6 @@
     <Compile Include="CompilerOptions\fsc\crossoptimize.fs" />
     <Compile Include="CompilerOptions\fsc\debug.fs" />
     <Compile Include="CompilerOptions\fsc\disableLanguageFeature.fs" />
-    <Compile Include="CompilerOptions\fsc\flaterrors.fs" />
     <Compile Include="CompilerOptions\fsc\FscOptionTests.fs" />
     <Compile Include="CompilerOptions\fsc\determinism\determinism.fs" />
     <Compile Include="CompilerOptions\fsc\highentropyva.fs" />