refactor(openai): inline chat tool choice validation

Author: Tom-Ryder

Sources/AgentRunKit/LLM/Providers/OpenAIChat/OpenAIChatToolChoiceValidation.swift

@@ -200,9 +200,15 @@ extension OpenAIClient {
 
         switch toolChoice {
         case .required:
-            try validateRequiredToolChoice(requestTools)
+            guard !requestTools.isEmpty else {
+                throw AgentError.llmError(.other("OpenAI Chat toolChoice.required requires at least one tool"))
+            }
         case let .function(name):
-            try validateRequestedFunctionChoice(name, functions: functions)
+            guard functions.contains(name) else {
+                throw AgentError.llmError(.other(
+                    "OpenAI Chat toolChoice.function requires tool '\(name)' in the request"
+                ))
+            }
         case let .custom(name):
             try validateRequestedCustomChoice(name, customs: customs, capabilities: capabilities)
         case let .allowedTools(_, tools):
@@ -214,6 +220,57 @@ extension OpenAIClient {
         return toolChoice
     }
 
+    private func validateRequestedCustomChoice(
+        _ name: String,
+        customs: Set<String>,
+        capabilities: OpenAIChatCapabilities
+    ) throws {
+        guard capabilities.supportsCustomTools else {
+            throw AgentError.llmError(.featureUnsupported(
+                provider: "openai-chat-\(capabilities.profile)",
+                feature: "custom tool choice"
+            ))
+        }
+        guard customs.contains(name) else {
+            throw AgentError.llmError(.other(
+                "OpenAI Chat toolChoice.custom requires custom tool '\(name)' in the request"
+            ))
+        }
+    }
+
+    private func validateAllowedTools(
+        _ tools: [OpenAIChatAllowedTool],
+        functions: Set<String>,
+        customs: Set<String>,
+        capabilities: OpenAIChatCapabilities
+    ) throws {
+        guard capabilities.profile == .firstParty else {
+            throw AgentError.llmError(.featureUnsupported(
+                provider: "openai-chat-\(capabilities.profile)",
+                feature: "allowed tools"
+            ))
+        }
+        guard !tools.isEmpty else {
+            throw AgentError.llmError(.other("OpenAI Chat allowed tools must not be empty"))
+        }
+        for tool in tools {
+            switch tool {
+            case let .function(name):
+                guard functions.contains(name) else {
+                    throw AgentError.llmError(.other(
+                        "OpenAI Chat allowed function tool '\(name)' is missing from the request"
+                    ))
+                }
+            case let .custom(name):
+                guard customs.contains(name) else {
+                    throw AgentError.llmError(.other(
+                        "OpenAI Chat allowed custom tool '\(name)' is missing from the request"
+                    ))
+                }
+            }
+        }
+    }
+
     func buildURLRequest(_ request: ChatCompletionRequest) throws -> URLRequest {
         let url = baseURL.appendingPathComponent(chatCompletionPath)
         var headers: [(String, String)] = []