fix: Improve error handling for FDv2 streaming (#1184)

BEGIN_COMMIT_OVERRIDE
fix: Improve error handling for FDv2 streaming
fix: Allow 0 status code to be handled by the streaming error filter.
END_COMMIT_OVERRIDE


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes reconnection/error classification logic in the streaming data
source, which could affect when clients retry vs surface interruptions.
Risk is moderate because it impacts core connectivity behavior but is
localized and guarded by existing error filters.
> 
> **Overview**
> Improves streaming error handling so *any numeric* `err.status`
(including `0`) is routed through the EventSource `errorFilter` rather
than being treated as a missing-status/network error.
> 
> Reduces noise and false failures by (1) not warning when FDv2
`'error'` events arrive without a body, and (2) only queuing a network
interruption on `es.onerror` when no numeric HTTP status is present
(letting the filter handle status-bearing errors).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
28ac8cf. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: kinyoklion

packages/sdk/browser/src/platform/DefaultBrowserEventSource.ts

@@ -97,7 +97,7 @@ export default class DefaultBrowserEventSource implements LDEventSource {
     // The event source may not produce a status. But the LaunchDarkly
     // polyfill can. If we can get the status, then we should stop retrying
     // on certain error codes.
-    if (err.status && typeof err.status === 'number' && !this._errorFilter(err)) {
+    if (typeof err.status === 'number' && !this._errorFilter(err)) {
       // If we encounter an unrecoverable condition, then we do not want to
       // retry anymore.
       return;

packages/shared/sdk-client/src/datasource/fdv2/StreamingFDv2Base.ts

@@ -188,7 +188,10 @@ export function createStreamingBase(config: {
         }
 
         if (!event?.data) {
-          config.logger?.warn(`Event from EventStream missing data for "${eventName}".`);
+          // Some events (e.g. 'error') may legitimately arrive without a body.
+          if (eventName !== 'error') {
+            config.logger?.warn(`Event from EventStream missing data for "${eventName}".`);
+          }
           return;
         }
 
@@ -277,8 +280,18 @@ export function createStreamingBase(config: {
         config.logger?.info('Closed LaunchDarkly stream connection');
       };
 
-      es.onerror = () => {
-        // Error handling is done by errorFilter.
+      es.onerror = (err?: HttpErrorResponse) => {
+        if (stopped) {
+          return;
+        }
+
+        if (err && typeof err.status === 'number') {
+          // This condition will be handled by the error filter.
+          return;
+        }
+        resultQueue.put(
+          interrupted(errorInfoFromNetworkError(err?.message ?? 'IO Error'), fdv1Fallback),
+        );
       };
 
       es.onopen = () => {