Issue: Inflexible API Versioning Logic in Foundation Models framework utilities

In the Foundation Models framework utilities package, the private method buildURLRequest in ChatCompletionsLanguageModel handles the construction of OpenAI-compatible API URLs:

private func buildURLRequest(for request: ChatCompletionRequest) throws -> URLRequest {
    let isVersioned = baseURL.pathComponents.contains("v1")
    let endpoint = isVersioned ? "/chat/completions" : "/v1/chat/completions"
    let url = baseURL.appendingPathComponent(endpoint)
    ...
  }

Problem

The current implementation hardcodes "v1" to determine if the baseURL already includes a version. This limits compatibility with API providers using alternative versioning schemes. For instance, Volcengine Ark uses "v3" in its Base URL, making it difficult to seamlessly integrate their services.

#Playground {
    let baseURL = URL(string: "https://ark.cn-beijing.volces.com/api/v3")!
    let modelName = "doubao-seed-2-0-mini-260428"

    let headers: [String : String] = [
        "Authorization" : "Bearer \(apiKey)"
    ]
    let model = ChatCompletionsLanguageModel(name: modelName, url: baseURL, additionalHeaders: headers)
    let session = LanguageModelSession(model: model)
    do {
        let result = try await session.respond(to: "Hello").content
    } catch {
        print(error.localizedDescription)
        // HTTP error with status code 404:
    }
}

#Playground {
    let baseURL = URL(string: "https://ark.cn-beijing.volces.com/api/v3/responses")!
    let modelName = "doubao-seed-2-0-mini-260428"

    let headers: [String : String] = [
        "Authorization" : "Bearer \(apiKey)"
    ]
    let model = ChatCompletionsLanguageModel(name: modelName, url: baseURL, additionalHeaders: headers)
    let session = LanguageModelSession(model: model)
    do {
        let result = try await session.respond(to: "Hello").content
    } catch {
        print(error.localizedDescription)
        /*
         HTTP error with status code 404:
         {"error":{"code":"InvalidAction","message":"The specified action is invalid: /api/v3/responses/v1/chat/completions Request id: 021784381168842fdfd2e3c33d5b6eddad55ac385080e727cab08","param":"","type":"NotFound"}}
         */
    }
}

Suggested Solution

To better accommodate different versioning conventions (e.g., v2, v3), we can leverage Swift's modern Regex (#/v\d+/#) to dynamically detect the version pattern in the path components. Here is a recommended update for the isVersioned check:

let isVersioned = baseURL.pathComponents.contains { component in
    component.wholeMatch(of: #/v\d+/#) != nil
}