[Repo Assist] Fix decodeILCustomAttribData: read correct byte width for non-int32 enum fixed args

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🤖 This PR was created by Repo Assist, an automated AI assistant.

Problem

decodeILCustomAttribData had a catch-all arm for enum-typed fixed arguments that always read 4 bytes regardless of the enum's actual underlying type:

| ILType.Value _ ->  (* assume it is an enumeration *)
    let n, sigptr = sigptr_get_i32 bytes sigptr
    (argty, box n), sigptr
```

Per ECMA-335 §II.23.3, custom attribute blobs encode enum fixed arguments using the **underlying type's width** — so a `byte`-backed enum encodes in 1 byte, an `int64`-backed one in 8 bytes. Always reading 4 bytes corrupts the stream offset for any subsequent attribute arguments when a non-`Int32` enum appears.

## Fix

Add a `resolveEnumUnderlyingILType: ILType -> ILType` parameter to `decodeILCustomAttribData`. At the call site in `txCustomAttributesDatum` (inside `TargetTypeDefinition`), the resolver:

1. Calls `txILType` to resolve the `ILType.Value` to a `System.Type`
2. Calls `Enum.GetUnderlyingType` to get the backing type
3. Maps back to the appropriate `ILType` (`typ_Byte`, `typ_Int16`, `typ_Int64`, etc.)
4. Falls back to `ilGlobals.typ_Int32` on any exception (preserving prior behaviour for unresolvable types)

The `ILType.Value _` arm now delegates to `parseVal underlyingTy sigptr`, which dispatches to the already-correct handler for the underlying primitive type, and returns `argty` (the original enum type) as the result type.

**For the common case (int32-backed enums):** the resolver returns `ilGlobals.typ_Int32` and behaviour is bit-for-bit identical to before. No regression risk for existing type providers.

**For non-int32 enums:** type providers consuming reference assemblies with `byte`, `int16`, `uint16`, `uint32`, `int64`, or `uint64` enum parameters on custom attributes will now decode correctly rather than silently mis-reading the blob.

## Test Status

```
Passed!  - Failed: 0, Passed: 110, Skipped: 0, Total: 110, Duration: 6 s

All 110 existing tests pass. This fix addresses the "Future Work" item noted in #465 (decodeILCustomAttribData enum-size bug).

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[dsyme]

/repo-assist we need tests added for this case - unit tests would be fine if they can be arranged, even better integration tests

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Commit pushed: 79de50a

Generated by Repo Assist

[github-actions]

🤖 This is an automated response from Repo Assist.

I've pushed a commit to this PR with the tests you asked for.

What was added (tests/BasicGenerativeProvisionTests.fs):

Three integration tests:

• decodeILCustomAttribData decodes byte-backed enum custom attribute correctly
• decodeILCustomAttribData decodes int16-backed enum custom attribute correctly
• decodeILCustomAttribData decodes int64-backed enum custom attribute correctly

Each test creates a generative type provider that attaches a custom attribute backed by a non-int32 enum, generates the assembly, then reads it back via tp.TargetContext.ReadRelatedAssembly(...). This exercises decodeILCustomAttribData directly (the runtime Assembly.Load path bypasses the IL decoder). The tests verify both that no exception is thrown and that the decoded argument value matches the encoded one.

Also fixed (src/ProvidedTypes.fs):

TargetTypeDefinition.GetEnumUnderlyingType() previously had a // TODO and always returned Int32. The new resolveEnumUnderlyingILType closure introduced by this PR calls Enum.GetUnderlyingType(resolvedType), which delegates to that method—so without fixing it, the resolver would always fall back to Int32 and the decoding bug would remain for target types. The fix reads the value__ field from the ILTypeDef to return the actual underlying type.

All 113 tests pass (110 pre-existing + 3 new).

Generated by Repo Assist for issue #472 · ◷

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