[Repo Assist] Fix decodeILCustomAttribData: resolve System.Type custom attribute arguments (#490)

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## Summary

Fixes a long-standing TODO in `decodeILCustomAttribData` where custom
attribute arguments of type `System.Type` were silently decoded as
`null` instead of the actual `Type` object.

## Root Cause

In ECMA-335, custom attribute constructors store `System.Type` arguments
as serialised strings (the assembly-qualified type name) in the
attribute blob — not as `TypeRef`/`TypeDef` table indices. The existing
code parsed the type name string correctly via `ILTypeSigParser`, but
then discarded the result and returned `null`:

```fsharp
// Before — TODO comments, always returned null
| None -> (argty, box null), sigptr // TODO: read System.Type attrs
| Some n ->
    let parser = ILTypeSigParser(n)
    parser.ParseType() |> ignore
    (argty, box null), sigptr  // TODO: read System.Type attributes
```

## Fix

1. Added a `resolveILType: ILType -> Type` parameter to
`decodeILCustomAttribData` so the caller provides a type resolver.
2. The `System.Type` branch now uses the parsed `ILType` and resolves it
via the provided resolver.
3. Graceful fallback to `null` if resolution fails (e.g. the type is not
present in the target context).
4. At the call site in `txCustomAttributesDatum`, passes `txILType
([||], [||])` — the existing target-assembly type translator — as the
resolver.

```fsharp
// After — resolves the type from the attribute blob
| None -> (argty, box null), sigptr
| Some n ->
    let parser = ILTypeSigParser(n)
    let ilty = parser.ParseType()
    let resolvedType = try resolveILType ilty with _ -> null
    (argty, box resolvedType), sigptr
```

## Impact

Custom attributes like `DebuggerTypeProxyAttribute` (constructor takes
`Type`) that are read from binary reference assemblies via the target
assembly reader now return the correct `Type` value instead of `null`.
This improves the accuracy of `GetCustomAttributesData()` for
target-context types.

## Test Status

✅ All 126 tests pass locally (`dotnet test
tests/FSharp.TypeProviders.SDK.Tests.fsproj -c Release`)

A dedicated test for this specific path would require finding a
reference assembly type with a `System.Type` custom attribute argument
and verifying it resolves non-null — feasible but deferred to a
follow-up as it requires navigating the target context fixture.




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---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
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Author: github-actions[bot]

src/ProvidedTypes.fs

@@ -6695,7 +6695,7 @@ module internal AssemblyReader =
                 let s, sigptr = sigptr_get_string len bytes sigptr
                 Some(s), sigptr
 
-        let decodeILCustomAttribData ilg (resolveEnumUnderlyingILType: ILType -> ILType) (ca: ILCustomAttribute) =
+        let decodeILCustomAttribData ilg (resolveEnumUnderlyingILType: ILType -> ILType) (resolveILType: ILType -> Type) (ca: ILCustomAttribute) =
             let bytes = ca.Data
             let sigptr = 0
             let bb0, sigptr = sigptr_get_byte bytes sigptr
@@ -6748,12 +6748,13 @@ module internal AssemblyReader =
                 | ILType.Boxed tspec when tspec.Namespace = USome "System" && tspec.Name = "Type" ->
                     let nOpt, sigptr = sigptr_get_serstring_possibly_null bytes sigptr
                     match nOpt with
-                    | None -> (argty, box null) , sigptr // TODO: read System.Type attrs
+                    | None -> (argty, box null), sigptr
                     | Some n ->
                     try
                         let parser = ILTypeSigParser(n)
-                        parser.ParseType() |> ignore
-                        (argty, box null) , sigptr // TODO: read System.Type attributes
+                        let ilty = parser.ParseType()
+                        let resolvedType = try resolveILType ilty with _ -> null
+                        (argty, box resolvedType), sigptr
                     with e ->
                         failwithf "decodeILCustomAttribData: error parsing type in custom attribute blob: %s" e.Message
                 | ILType.Boxed tspec when tspec.Namespace = USome "System" && tspec.Name = "Object" ->
@@ -7697,7 +7698,7 @@ namespace ProviderImplementation.ProvidedTypes
                          else ilGlobals.typ_Int32
                      with _ -> ilGlobals.typ_Int32
                  | _ -> ilGlobals.typ_Int32
-             let args, namedArgs = decodeILCustomAttribData ilGlobals resolveEnumUnderlyingILType inp
+             let args, namedArgs = decodeILCustomAttribData ilGlobals resolveEnumUnderlyingILType (txILType ([| |], [| |])) inp
              { new CustomAttributeData () with
                 member __.Constructor =  txILConstructorRef inp.Method.MethodRef
                 member __.ConstructorArguments = [| for arg in args -> txCustomAttributesArg arg |] :> IList<_>