[Repo Assist] Fix decodeILCustomAttribData: resolve System.Type custom attribute arguments

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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:

// 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.

// 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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