Fix server handler Sendability warnings (#156)

Fix server handler Sendability warnings

### Motivation

Fixes a warning emitted for each operation method in generated server code, which looked like:

```
Converting non-sendable function value to '@sendable (APIHandler) -> ((Operations.listPets.Input) async throws -> Operations.listPets.Output)' may introduce data races
```

### Modifications

Instead of passing the function itself, we now invoke it in a closure.

### Result

The warning has gone away.

### Test Plan

Adapted integration tests.


Reviewed by: simonjbeaumont

Builds:
     ✔︎ pull request validation (5.8) - Build finished. 
     ✔︎ pull request validation (5.9) - Build finished. 
     ✔︎ pull request validation (docc test) - Build finished. 
     ✔︎ pull request validation (integration test) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#156

Sources/_OpenAPIGeneratorCore/Translator/ServerTranslator/translateServerMethod.swift

@@ -206,8 +206,17 @@ extension ServerFileTranslator {
         )
         let methodArg: FunctionArgumentDescription = .init(
             label: "using",
-            expression: .identifier(Constants.Server.Universal.apiHandlerName)
-                .dot(description.methodName)
+            expression: .closureInvocation(
+                body: [
+                    .expression(
+                        .identifier(Constants.Server.Universal.apiHandlerName)
+                            .dot(description.methodName)
+                            .call([
+                                .init(label: nil, expression: .identifier("$0"))
+                            ])
+                    )
+                ]
+            )
         )
         let deserializerArg: FunctionArgumentDescription = .init(
             label: "deserializer",

Tests/OpenAPIGeneratorReferenceTests/Resources/ReferenceSources/Petstore/Server.swift

@@ -69,7 +69,7 @@ fileprivate extension UniversalServer where APIHandler: APIProtocol {
             request: request,
             with: metadata,
             forOperation: Operations.listPets.id,
-            using: APIHandler.listPets,
+            using: { APIHandler.listPets($0) },
             deserializer: { request, metadata in let path: Operations.listPets.Input.Path = .init()
                 let query: Operations.listPets.Input.Query = .init(
                     limit: try converter.getOptionalQueryItemAsText(
@@ -179,7 +179,7 @@ fileprivate extension UniversalServer where APIHandler: APIProtocol {
             request: request,
             with: metadata,
             forOperation: Operations.createPet.id,
-            using: APIHandler.createPet,
+            using: { APIHandler.createPet($0) },
             deserializer: { request, metadata in
                 try converter.validateContentTypeIfPresent(
                     in: request.headerFields,
@@ -279,7 +279,7 @@ fileprivate extension UniversalServer where APIHandler: APIProtocol {
             request: request,
             with: metadata,
             forOperation: Operations.probe.id,
-            using: APIHandler.probe,
+            using: { APIHandler.probe($0) },
             deserializer: { request, metadata in let path: Operations.probe.Input.Path = .init()
                 let query: Operations.probe.Input.Query = .init()
                 let headers: Operations.probe.Input.Headers = .init()
@@ -314,7 +314,7 @@ fileprivate extension UniversalServer where APIHandler: APIProtocol {
             request: request,
             with: metadata,
             forOperation: Operations.updatePet.id,
-            using: APIHandler.updatePet,
+            using: { APIHandler.updatePet($0) },
             deserializer: { request, metadata in
                 try converter.validateContentTypeIfPresent(
                     in: request.headerFields,
@@ -389,7 +389,7 @@ fileprivate extension UniversalServer where APIHandler: APIProtocol {
             request: request,
             with: metadata,
             forOperation: Operations.uploadAvatarForPet.id,
-            using: APIHandler.uploadAvatarForPet,
+            using: { APIHandler.uploadAvatarForPet($0) },
             deserializer: { request, metadata in
                 try converter.validateContentTypeIfPresent(
                     in: request.headerFields,