feat: add test hook button for notification hooks

[caco3]

Adds a Test button to every hook form so users can immediately verify a hook (e.g. Telegram, Discord, Slack) with a test message before saving:

What's changed

• Adds a TestHook RPC to the backend.
• TestHook executes the provided hook with synthetic HookVars and returns an error if the hook fails.
• TestHook uses a real TaskRunner via orchestrator.NewReadOnlyTaskRunner. A readOnlyTaskRunner decorator wraps the real runner and no-ops state-mutating methods (CreateOperation, UpdateOperation, DeleteOperation, ScheduleTask, LogrefWriter) so the test cannot create operations, schedule tasks, or write log refs.
• Adds a play button to each hook form in the UI that serializes the current hook, calls testHook, and shows a success or error toast.

Technical details

• proto/v1/service.proto: adds the TestHook RPC.
• gen/go/v1/service.*.go: generated client/server code for the new RPC.
• internal/orchestrator/taskrunnerimpl.go: adds readOnlyTaskRunner and the NewReadOnlyTaskRunner(orchestrator, task) constructor.
• internal/api/backresthandler.go: implements TestHook by creating a minimal tasks.GenericOneoffTask and running the hook through orchestrator.NewReadOnlyTaskRunner.

Notes

• This is mainly AI driven; my Go skills are very limited, but I reviewed and tested the Telegram hook.
• I only tested the Telegram hook as I have no infrastructure for the other services.

[caco3]

There are many open issues which will benefit from this feature:

• notify-send hook command fails on Ubuntu #919
• Help to send notifications contents to a webhook #868

[garethgeorge]

I think it'd be fine to use a real [taskrunnerimpl](https://github.com/garethgeorge/backrest/blob/main/internal/orchestrator/taskrunnerimpl.go] here.

Let's introduce a ReadOnlyTaskRunnerDecorator in that same file which wraps a task runner and no-ops methods that mutate state

1. CreateOperation
2. DeleteOperation
3. ScheduleTask
4. LogrefWriter

wdyt?

[garethgeorge]

Thanks for the interest in contributing this -- agree this is much wanted and its a great contribution!

I think it'd actually be reasonable to inject a real but decorated taskrunnerimpl here and decorate it.

[caco3]

@garethgeorge Good call — using a real task runner is much cleaner. I added the ReadOnlyTaskRunnerDecorator and switched TestHook to use it.

What changed

internal/orchestrator/taskrunnerimpl.go

• Added readOnlyTaskRunner, a decorator that wraps a real taskRunnerImpl.
• It overrides the state-mutating methods to no-op:
  • CreateOperation
  • UpdateOperation
  • DeleteOperation
  • ScheduleTask
  • LogrefWriter
• Every other method (Config, InstanceID, GetRepo, GetPlan, Logger, etc.) is delegated to the real runner.
• Added NewReadOnlyTaskRunner(orchestrator, task) as a helper constructor.

internal/api/backresthandler.go

• Replaced the hand-written testHookRunner stub with a real read-only runner:

  testTask := &tasks.GenericOneoffTask{
      BaseTask: tasks.BaseTask{
          TaskName: "test hook",
          TaskType: "test",
      },
      Do: func(ctx context.Context, st tasks.ScheduledTask, runner tasks.TaskRunner) error { return nil },
  }
  runner := orchestrator.NewReadOnlyTaskRunner(s.orchestrator, testTask)

• Removed the testHookRunner and nopWriteCloser stub types.