Implement optional discussion mode

Sources/MusicAssistantCore/Services/AI/AIService.swift

@@ -2,10 +2,21 @@ import Foundation
 
 public protocol AIService: Sendable {
     func generateSongProject(from request: SongProjectGenerationRequest) async throws -> SongProjectGenerationResult
+    func discussSongProject(from request: SongProjectDiscussionRequest) async throws -> SongProjectDiscussionResult
     func reviseLyrics(from request: LyricsRevisionRequest) async throws -> LyricsRevisionResult
     func proposeProjectUpdate(from request: SongProjectUpdateRequest) async throws -> SongProjectUpdateResult
 }
 
+public extension AIService {
+    func discussSongProject(from request: SongProjectDiscussionRequest) async throws -> SongProjectDiscussionResult {
+        throw AIServiceCapabilityError.discussionNotSupported
+    }
+}
+
+public enum AIServiceCapabilityError: Error, Equatable, Sendable {
+    case discussionNotSupported
+}
+
 public struct AIConversationMessage: Equatable, Identifiable, Sendable {
     public let id: String
     public var role: AIConversationRole
@@ -82,6 +93,26 @@ public struct SongProjectGenerationResult: Equatable, Sendable {
     }
 }
 
+public struct SongProjectDiscussionRequest: Equatable, Sendable {
+    public var context: AIRequestContext
+    public var project: SongProject
+
+    public init(context: AIRequestContext, project: SongProject) {
+        self.context = context
+        self.project = project
+    }
+}
+
+public struct SongProjectDiscussionResult: Equatable, Sendable {
+    public var questions: [String]
+    public var notes: [String]
+
+    public init(questions: [String], notes: [String] = []) {
+        self.questions = questions
+        self.notes = notes
+    }
+}
+
 public struct LyricsRevisionRequest: Equatable, Sendable {
     public var context: AIRequestContext
     public var project: SongProject