Add support for customizable timeout for AI-generated commit messages

[seeseemelk]

When using Ollama to generate commit messages, some larger files take a bit longer to generate the commit message for. For especially large files, this can result in a timeout:

Failed to generate commit message: System.AggregateException: One or more errors occurred. (A task was canceled.)
 ---> System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at System.Threading.Tasks.Task.GetExceptions(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait(CancellationToken cancellationToken)
   at SourceGit.Models.OpenAIService.Chat(String prompt, String question, CancellationToken cancellation)
   at SourceGit.Commands.GenerateCommitMessage.GenerateChangeSummary(Change change)
   at SourceGit.Commands.GenerateCommitMessage.Result()
   at SourceGit.Views.AIAssistant.<OnOpened>b__2_0()
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()
--- End of stack trace from previous location ---

   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait(CancellationToken cancellationToken)
   at SourceGit.Models.OpenAIService.Chat(String prompt, String question, CancellationToken cancellation)
   at SourceGit.Commands.GenerateCommitMessage.GenerateChangeSummary(Change change)
   at SourceGit.Commands.GenerateCommitMessage.Result()

It would be nice if this timeout can be configured or even turned off completely.

[love-linger]

The OpenAI integration will be replaced with a new implementation after openai/openai-dotnet#21 is merged. You can test it from branch feat-openai.