[Repo Assist] Fix GetEnumUnderlyingType for non-Int32 enums + tests for byte/int64 enums

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Two related improvements to enum support in the type provider SDK.

Fix: TargetTypeDefinition.GetEnumUnderlyingType() (Task 5 – Coding Improvement)

Before: TargetTypeDefinition.GetEnumUnderlyingType() always returned System.Int32, regardless of the actual enum backing type in the reference assembly:

// TODO: in theory the assumption of "Int32" is not accurate for all enums,
// however in practice .NET only uses enums with backing field Int32
txILType ([| |], [| |]) ilGlobals.typ_Int32

The comment was incorrect — .NET (and F#/C#) fully support enums backed by byte, sbyte, int16, uint16, int32, uint32, int64, and uint64.

After: Looks up the value__ special field that the compiler emits on every enum IL type definition and returns its type. Falls back to Int32 only if the field is absent (which would indicate a malformed binary):

let valueField = inp.Fields.Entries |> Array.tryFind (fun f -> f.Name = "value__")
match valueField with
| Some f -> txILType ([| |], [| |]) f.FieldType
| None -> txILType ([| |], [| |]) ilGlobals.typ_Int32
```

This affects type providers that read reference assemblies containing non-`int32` enums and need to accurately reflect their underlying type (e.g., interop scenarios with `byte` flag enums, or `int64` discriminated enums).

## Tests: Generative enums with `byte` and `int64` underlying types (Task 9 – Testing Improvements)

Added two new type provider fixtures and two new `[(Fact)]` tests in `GenerativeEnumsProvisionTests.fs`:

| Test | Provider | Underlying type | Notable case |
|------|----------|-----------------|--------------|
| `Generative enum with byte underlying type is generated correctly` | `GenerativeByteEnumsProvider` | `byte` (`uint8`) | Verifies 0, 1, 255 encoded correctly |
| `Generative enum with int64 underlying type is generated correctly` | `GenerativeInt64EnumsProvider` | `int64` | Uses `3_000_000_000L` (> `Int32.MaxValue`) to confirm 64-bit path |

Both tests verify:
- The generated assembly loads without error
- `IsEnum` is true on the generated type
- `GetEnumUnderlyingType()` returns the expected type
- `Enum.GetValues` and `Enum.GetNames` return the correct values

## Test Status

```
Passed!  - Failed: 0, Passed: 112, Skipped: 0, Total: 112, Duration: 6 s

108 existing tests + 4 new tests, all passing.

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