[Repo Assist] refactor: add save-based caching to GetField/GetEvent/GetNestedType; use Dictionary in ILNestedExportedTypesAndForwarders (#498)

🤖 *This is an automated PR from Repo Assist.*

## Summary

Two complementary consistency/performance improvements to
`ProvidedTypeDefinition` and `ILNestedExportedTypesAndForwarders`.

### 1. `save`-based caching for `GetField`, `GetEvent`, `GetNestedType`

**Before**: these three overrides used `Array.filter` / `Array.tryFind`
for O(n) linear scans on every call:

```fsharp
override this.GetField(name, bindingFlags) =
    let xs = this.GetFields bindingFlags |> Array.filter (fun m -> m.Name = name)
    if xs.Length > 0 then xs.[0] else null
```

**After**: they now use the same `save`-based dictionary cache that
`GetMethodImpl` and `GetPropertyImpl` already use — building a `name →
member` dictionary once per `bindingFlags` combination and doing O(1)
lookups thereafter:

```fsharp
override this.GetField(name, bindingFlags) =
    let table =
        save (bindingFlags ||| BindingFlags.GetField) (fun () ->
            this.GetFields bindingFlags |> Seq.map (fun f -> f.Name, f) |> dict)
    match table.TryGetValue name with | true, f -> f | false, _ -> null
```

Discriminator bits chosen to avoid cache-key collisions with existing
`save` calls:
| Member | Discriminator bit |
|--------|-------------------|
| `GetMethodImpl` | `BindingFlags.InvokeMethod` (0x100) — existing |
| `GetPropertyImpl` | `BindingFlags.GetProperty` (0x1000) — existing |
| `GetField` | `BindingFlags.GetField` (0x400) — new |
| `GetEvent` | `BindingFlags.SetField` (0x800) — new |
| `GetNestedType` | `BindingFlags.SetProperty` (0x2000) — new |

The stale `//save ("field1", ...)` / `//save ("event1", ...)` / `//save
("nested1", ...)` comments (from a previous refactor attempt) are also
removed to reduce noise.

### 2. `ILNestedExportedTypesAndForwarders`: `Map` → `Dictionary`

**Before**: used a functional `Map` (balanced BST, O(log n) lookup)
built via `Array.fold`:
```fsharp
let lmap = lazy ((Map.empty, larr.Force()) ||> Array.fold (fun m x -> m.Add(x.Name, x)))
member __.TryFindByName nm = lmap.Force().TryFind nm
```

**After**: consistent with the adjacent `ILTypeDefs` and
`ILExportedTypesAndForwarders` types which already use `Dictionary`:
```fsharp
let lmap = lazy (
    let m = Dictionary()
    for x in larr.Force() do m.[x.Name] <- x
    m)
member __.TryFindByName nm = match lmap.Force().TryGetValue nm with true, v -> Some v | false, _ -> None
```

## Test Status

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




> Generated by 🌈 Repo Assist, see [workflow
run](https://github.com/fsprojects/FSharp.TypeProviders.SDK/actions/runs/23990843787).
[Learn
more](https://github.com/githubnext/agentics/blob/main/docs/repo-assist.md).
>
> To install this [agentic
workflow](https://github.com/githubnext/agentics/blob/7ee2b60744abf71b985bead4599640f165edcd93/workflows/repo-assist.md),
run
> ```
> gh aw add
githubnext/agentics@7ee2b60
> ```

<!-- gh-aw-agentic-workflow: Repo Assist, engine: copilot, model: auto,
id: 23990843787, workflow_id: repo-assist, run:
https://github.com/fsprojects/FSharp.TypeProviders.SDK/actions/runs/23990843787
-->

<!-- gh-aw-workflow-id: repo-assist -->

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

src/ProvidedTypes.fs

@@ -1619,43 +1619,41 @@ and ProvidedTypeDefinition(isTgt: bool, container:TypeContainer, className: stri
         if xs.Length > 0 then xs.[0] else null
 
     override __.GetMethodImpl(name, bindingFlags, _binderBinder, _callConvention, _types, _modifiers): MethodInfo =
-        (//save ("methimpl", bindingFlags, Some name) (fun () -> 
-            // This is performance critical for large spaces of provided methods and properties
-            // Save a table of the methods grouped by name
-            let table = 
-                save (bindingFlags ||| BindingFlags.InvokeMethod) (fun () -> 
-                    let methods = this.GetMethods bindingFlags
-                    methods |> Seq.groupBy (fun m -> m.Name) |> Seq.map (fun (k, v) -> k, Seq.toArray v) |> dict)
-                
-            let xs = match table.TryGetValue name with | true, tn -> tn | false, _ -> [| |]
-            //let xs = this.GetMethods bindingFlags |> Array.filter (fun m -> m.Name = name)
-            if xs.Length > 1 then failwithf "GetMethodImpl. not support overloads, name = '%s', methods - '%A', callstack = '%A'" name xs Environment.StackTrace
-            if xs.Length > 0 then xs.[0] else null)
+        // This is performance critical for large spaces of provided methods and properties.
+        // Build a name→methods table once per bindingFlags combination and cache it.
+        let table =
+            save (bindingFlags ||| BindingFlags.InvokeMethod) (fun () ->
+                let methods = this.GetMethods bindingFlags
+                methods |> Seq.groupBy (fun m -> m.Name) |> Seq.map (fun (k, v) -> k, Seq.toArray v) |> dict)
+        let xs = match table.TryGetValue name with | true, tn -> tn | false, _ -> [| |]
+        if xs.Length > 1 then failwithf "GetMethodImpl. not support overloads, name = '%s', methods - '%A', callstack = '%A'" name xs Environment.StackTrace
+        if xs.Length > 0 then xs.[0] else null
 
     override this.GetField(name, bindingFlags) =
-        (//save ("field1", bindingFlags, Some name) (fun () -> 
-            let xs = this.GetFields bindingFlags |> Array.filter (fun m -> m.Name = name)
-            if xs.Length > 0 then xs.[0] else null)
+        let table =
+            save (bindingFlags ||| BindingFlags.GetField) (fun () ->
+                this.GetFields bindingFlags |> Seq.map (fun f -> f.Name, f) |> dict)
+        match table.TryGetValue name with | true, f -> f | false, _ -> null
 
     override __.GetPropertyImpl(name, bindingFlags, _binder, _returnType, _types, _modifiers) =
-        (//save ("prop1", bindingFlags, Some name) (fun () -> 
-            let table = 
-                save (bindingFlags ||| BindingFlags.GetProperty) (fun () -> 
-                    let methods = this.GetProperties bindingFlags
-                    methods |> Seq.groupBy (fun m -> m.Name) |> Seq.map (fun (k, v) -> k, Seq.toArray v) |> dict)
-            let xs = match table.TryGetValue name with | true, tn -> tn | false, _ -> [| |]
-            //let xs = this.GetProperties bindingFlags |> Array.filter (fun m -> m.Name = name)
-            if xs.Length > 0 then xs.[0] else null)
+        let table =
+            save (bindingFlags ||| BindingFlags.GetProperty) (fun () ->
+                let methods = this.GetProperties bindingFlags
+                methods |> Seq.groupBy (fun m -> m.Name) |> Seq.map (fun (k, v) -> k, Seq.toArray v) |> dict)
+        let xs = match table.TryGetValue name with | true, tn -> tn | false, _ -> [| |]
+        if xs.Length > 0 then xs.[0] else null
 
     override __.GetEvent(name, bindingFlags) =
-        (//save ("event1", bindingFlags, Some name) (fun () -> 
-            let xs = this.GetEvents bindingFlags |> Array.filter (fun m -> m.Name = name)
-            if xs.Length > 0 then xs.[0] else null)
+        let table =
+            save (bindingFlags ||| BindingFlags.SetField) (fun () ->
+                this.GetEvents bindingFlags |> Seq.map (fun e -> e.Name, e) |> dict)
+        match table.TryGetValue name with | true, e -> e | false, _ -> null
 
     override __.GetNestedType(name, bindingFlags) =
-        (//save ("nested1", bindingFlags, Some name) (fun () -> 
-            let xs = this.GetNestedTypes bindingFlags |> Array.filter (fun m -> m.Name = name)
-            if xs.Length > 0 then xs.[0] else null)
+        let table =
+            save (bindingFlags ||| BindingFlags.SetProperty) (fun () ->
+                this.GetNestedTypes bindingFlags |> Seq.map (fun t -> t.Name, t) |> dict)
+        match table.TryGetValue name with | true, t -> t | false, _ -> null
 
     override __.GetInterface(name, ignoreCase) =
         let sc = if ignoreCase then StringComparison.OrdinalIgnoreCase else StringComparison.Ordinal
@@ -3133,9 +3131,12 @@ module internal AssemblyReader =
         override x.ToString() = "nested fwd " + x.Name
 
     and ILNestedExportedTypesAndForwarders(larr:Lazy<ILNestedExportedType[]>) =
-        let lmap = lazy ((Map.empty, larr.Force()) ||> Array.fold (fun m x -> m.Add(x.Name, x)))
+        let lmap = lazy (
+            let m = Dictionary()
+            for x in larr.Force() do m.[x.Name] <- x
+            m)
         member __.Entries = larr.Force()
-        member __.TryFindByName nm = lmap.Force().TryFind nm
+        member __.TryFindByName nm = match lmap.Force().TryGetValue nm with true, v -> Some v | false, _ -> None
 
     and [<NoComparison; NoEquality>]
         ILExportedTypeOrForwarder =