[Repo Assist] Fix generative delegate type support; implement GetInterface on ProvidedTypeDefinition

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Summary

Two related fixes for correctness gaps in the SDK.

---

Bug fix: Generative delegate types were unsupported (Task 3)

Attempting to create a generative ProvidedTypeDefinition that extends System.MulticastDelegate would previously fail at assembly-writing time with:

"The provided method is not marked as an abstract method; therefore, it should define an implementation."

The root cause: .NET delegate types are special. Their .ctor(object, nativeint) and Invoke/BeginInvoke/EndInvoke methods must carry MethodImplAttributes.Runtime | MethodImplAttributes.Managed — the CLR synthesises their bodies at JIT time, and no IL body is ever written in the binary. The old assembly writer had no path for this case.

Changes in ProvidedTypes.fs:

• Add ILMethodBuilder.SetImplementationFlags to expose impl-attribute control
• In assembly writer phase 2, detect delegate types (BaseType.FullName = "System.MulticastDelegate") and set Runtime|Managed impl flags on all their constructors and methods
• In assembly writer phase 3, skip IL body emission for delegate type constructors and methods (the CLR synthesises them)

Example usage that now works:

let myHandler = ProvidedTypeDefinition("MyHandler", Some typeof(System.MulticastDelegate), isErased = false)
myHandler.AddMember(
    ProvidedConstructor(
        [ ProvidedParameter("object", typeof(obj))
          ProvidedParameter("method", typeof(nativeint)) ],
        invokeCode = fun _ -> <@@ () @@>))          // invokeCode is ignored for delegate types
myHandler.AddMember(
    ProvidedMethod("Invoke",
        [ ProvidedParameter("sender", typeof(obj))
          ProvidedParameter("e", typeof(EventArgs)) ],
        typeof(Void)))
```

---

### Coding improvement: `GetInterface` now works instead of throwing (Task 5)

`ProvidedTypeDefinition.GetInterface(name, ignoreCase)` and `TargetTypeDefinition.GetInterface(name, ignoreCase)` previously threw `NotSupportedException`. They now search through `GetInterfaces()`, matching on `Name` (short name) or `FullName` (dotted name), consistent with how `TypeDelegator` implements this in .NET.

---

### Tests

Added `tests/GenerativeDelegateTests.fs` with **5 new tests**:

| Test | What it verifies |
|------|-----------------|
| `Generative delegate type is present in generated assembly` | Type exists with `MulticastDelegate` as base |
| `Generative delegate type has correct constructor` | `.ctor(object, nativeint)` present with correct param types |
| `Generative delegate Invoke method has correct signature` | `Invoke(object, EventArgs): void` |
| `Generative delegate with value return type has correct Invoke signature` | `Invoke(int, int): int` |
| `Multiple delegate types can coexist in one container` | Both delegate types present; both have `MulticastDelegate` base |

## Test Status

All **122 tests pass** (117 pre-existing + 5 new), `net8.0`.

```
Passed!  - Failed: 0, Passed: 122, Skipped: 0, Total: 122

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[dsyme]

/repo-assist Fix these CI errors please

/home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK> /usr/share/dotnet/dotnet build src/FSharp.TypeProviders.SDK.fsproj --configuration Release /nodeReuse:False /clp:ForceConsoleColor (In: false, Out: false, Err: false)
MSBuild version 17.8.49+7806cbf7b for .NET
Determining projects to restore...
Restored /home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/FSharp.TypeProviders.SDK.fsproj (in 808 ms).
Error: /home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/ProvidedTypes.fs(1664,30): error FS0001: This expression was expected to have type� 'string' �but here has type� 'char' [/home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/FSharp.TypeProviders.SDK.fsproj::TargetFramework=netstandard2.0]
Error: /home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/ProvidedTypes.fs(8161,34): error FS0001: This expression was expected to have type� 'string' �but here has type� 'char' [/home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/FSharp.TypeProviders.SDK.fsproj::TargetFramework=netstandard2.0]
FSharp.TypeProviders.SDK -> /home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/bin/Release/net8.0/FSharp.TypeProviders.SDK.dll

Build FAILED.

Error: /home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/ProvidedTypes.fs(1664,30): error FS0001: This expression was expected to have type� 'string' �but here has type� 'char' [/home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/FSharp.TypeProviders.SDK.fsproj::TargetFramework=netstandard2.0]
Error: /home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/ProvidedTypes.fs(8161,34): error FS0001: This expression was expected to have type� 'string' �but here has type� 'char' [/home/runner/work/FSharp.TypeProviders.SDK/FSharp.TypeProviders.SDK/src/FSharp.TypeProviders.SDK.fsproj::TargetFramework=netstandard2.0]
0 Warning(s)
2 Error(s)

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Commit pushed: b1d5c7f

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