chore: Ensure transition to falling back state based on synchronizer failure. (#207)

Addresses the possible race condition raised in the previous
initialization PR.

<img width="772" height="350" alt="image"
src="https://github.com/user-attachments/assets/464249fb-acc6-486d-bbfc-3ea3ddee1bdf"
/>

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Ensure InitializationTracker transitions to `FallingBack` when
synchronizers turn `Off` with FDv1 fallback reason, and allow fallback
from `NoData`.
> 
> - **SDK Server (FDv2 InitializationTracker)**:
> - When `DataSourceCategory.Synchronizers` report `Off` with
`ErrorInfo.FDv1Fallback`, transition to `FallingBack`, then handle
`SynchronizersExhausted`.
> - Permit `Action.FallingBack` to transition `NoData` → `FallingBack`.
> - For `FallbackSynchronizers` entering `Initializing`, also transition
to `FallingBack`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
0fcbc78. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: kinyoklion

pkgs/sdk/server/src/Internal/FDv2DataSources/FDv2DataSource.InitializationTracker.cs

@@ -142,6 +142,9 @@ private void DetermineState(Action action)
                                 case Action.DataReceived:
                                     _state = State.Data;
                                     break;
+                                case Action.FallingBack:
+                                    _state = State.FallingBack;
+                                    break;
                                 case Action.InitializersExhausted:
                                     _initializersRemain = false;
                                     _state = State.InitializersExhausted;
@@ -254,7 +257,16 @@ public void UpdateStatus(DataSourceState newState, DataSourceStatus.ErrorInfo? n
                     }
                     case DataSourceCategory.Synchronizers when newState == DataSourceState.Off:
                     {
+                        // Currently, FDv1 fallback happens in the synchronizers group. If something from that group
+                        // reports Off, with a reason indicating that it should fallback to v1, then we can
+                        // transition to that state.
+                        if (newError.HasValue && newError.Value.FDv1Fallback)
+                        {
+                            DetermineState(Action.FallingBack);
+                        }
+                        
                         DetermineState(Action.SynchronizersExhausted);
+
                         break;
                     }
                     case DataSourceCategory.FallbackSynchronizers when newState == DataSourceState.Off:
@@ -264,6 +276,8 @@ public void UpdateStatus(DataSourceState newState, DataSourceStatus.ErrorInfo? n
                     }
                     case DataSourceCategory.FallbackSynchronizers when newState == DataSourceState.Initializing:
                     {
+                        // In the normal flow this will have be already handled via the `Off` state of the
+                        // synchronizers. This is purely redundant in the current implementation.
                         DetermineState(Action.FallingBack);
                         break;
                     }