[client] Restores lost backup.Reset from #4935 (#6883)

## Describe your changes

Restore the management gRPC client's backoff reset after a stream
connects.

This is a regression: PR #4935 originally added this reset; it was then
lost in
commit `58daa674e` during the `withMgmtStream` refactor (Sync/Job
unification).

Compared to before the reset was done AFTER receiveEvents, whereas 
now it's done before. 

Now should cover for Sync and Job (which didn't exist in #4935. Hold for
long living stream that breaks.

Reset prevents two things

1. long living conns going wrong from waiting a long backoff time window
before to drive reconn
2. past MaxElapsedTime (3months) long lived conns failures from being
treated as "unrecoverable" (hence triggering an full restart of the
engine)

## Issue ticket number and link

Internal support case (customer agents lost data-plane connectivity
during a
management maintenance/release window). No public issue. Regression
introduced
in commit `58daa674e`, which removed the `backOff.Reset()` added by PR
#4935.

## Stack

<!-- branch-stack -->

### Checklist
- [x] Is it a bug fix
- [ ] Is a typo/documentation fix
- [ ] Is a feature enhancement
- [ ] It is a refactor
- [ ] Created tests that fail without the change (if possible)

> By submitting this pull request, you confirm that you have read and
agree to the terms of the [Contributor License
Agreement](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTOR_LICENSE_AGREEMENT.md).

## Documentation
Select exactly one:

- [ ] I added/updated documentation for this change
- [x] Documentation is **not needed** for this change (explain why)

Internal reconnection-behavior fix in the management gRPC client. No
public
NetBird CLI, API, or configuration surface changes.

### Docs PR URL (required if "docs added" is checked)
Paste the PR link from https://github.com/netbirdio/docs here:

N/A

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
  * Improved management stream retry behavior.
* Retry delays now reset after a stream is successfully established,
helping subsequent connection attempts recover more quickly.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

shared/management/client/grpc.go

@@ -187,16 +187,16 @@ func (c *GrpcClient) ready() bool {
 // Sync wraps the real client's Sync endpoint call and takes care of retries and encryption/decryption of messages
 // Blocking request. The result will be sent via msgHandler callback function
 func (c *GrpcClient) Sync(ctx context.Context, sysInfo *system.Info, msgHandler func(msg *proto.SyncResponse) error) error {
-	return c.withMgmtStream(ctx, func(ctx context.Context, serverPubKey wgtypes.Key) error {
+	return c.withMgmtStream(ctx, func(ctx context.Context, serverPubKey wgtypes.Key, backOff backoff.BackOff) error {
-		return c.handleSyncStream(ctx, serverPubKey, sysInfo, msgHandler)
+		return c.handleSyncStream(ctx, serverPubKey, sysInfo, msgHandler, backOff)
 	})
 }
 
 // Job wraps the real client's Job endpoint call and takes care of retries and encryption/decryption of messages
 // Blocking request. The result will be sent via msgHandler callback function
 func (c *GrpcClient) Job(ctx context.Context, msgHandler func(msg *proto.JobRequest) *proto.JobResponse) error {
-	return c.withMgmtStream(ctx, func(ctx context.Context, serverPubKey wgtypes.Key) error {
+	return c.withMgmtStream(ctx, func(ctx context.Context, serverPubKey wgtypes.Key, backOff backoff.BackOff) error {
-		return c.handleJobStream(ctx, serverPubKey, msgHandler)
+		return c.handleJobStream(ctx, serverPubKey, msgHandler, backOff)
 	})
 }
 
@@ -204,7 +204,7 @@ func (c *GrpcClient) Job(ctx context.Context, msgHandler func(msg *proto.JobRequ
 // It takes care of retries, connection readiness, and fetching server public key.
 func (c *GrpcClient) withMgmtStream(
 	ctx context.Context,
-	handler func(ctx context.Context, serverPubKey wgtypes.Key) error,
+	handler func(ctx context.Context, serverPubKey wgtypes.Key, backOff backoff.BackOff) error,
 ) error {
 	backOff := defaultBackoff(ctx)
 	operation := func() error {
@@ -224,7 +224,7 @@ func (c *GrpcClient) withMgmtStream(
 			return err
 		}
 
-		return handler(ctx, *serverPubKey)
+		return handler(ctx, *serverPubKey, backOff)
 	}
 
 	err := backoff.Retry(operation, backOff)
@@ -239,6 +239,7 @@ func (c *GrpcClient) handleJobStream(
 	ctx context.Context,
 	serverPubKey wgtypes.Key,
 	msgHandler func(msg *proto.JobRequest) *proto.JobResponse,
+	backOff backoff.BackOff,
 ) error {
 	ctx, cancelStream := context.WithCancel(ctx)
 	defer cancelStream()
@@ -256,6 +257,19 @@ func (c *GrpcClient) handleJobStream(
 
 	log.Debug("job stream handshake sent successfully")
 
+	// The stream is up, so reset the backoff. This matters for two reasons,
+	// both caused by the backoff lib not resetting its state on a successful
+	// connection:
+	//  1. Without a reset, after a connect followed by an error the next retry
+	//     starts from the accumulated (large) interval instead of retrying
+	//     promptly, delaying reconnection.
+	//  2. Worse, once the accumulated elapsed time exceeds MaxElapsedTime, the
+	//     next stream error makes NextBackOff() return Stop, so the retry loop
+	//     exits immediately. That error is then mislabeled unrecoverable and
+	//     bubbles up to trigger a full engine restart / data-plane teardown
+	//     instead of a silent reconnection.
+	backOff.Reset()
+
 	// Main loop: receive, process, respond
 	for {
 		jobReq, err := c.receiveJobRequest(ctx, stream, serverPubKey)
@@ -371,7 +385,7 @@ func (c *GrpcClient) sendJobResponse(
 	return nil
 }
 
-func (c *GrpcClient) handleSyncStream(ctx context.Context, serverPubKey wgtypes.Key, sysInfo *system.Info, msgHandler func(msg *proto.SyncResponse) error) error {
+func (c *GrpcClient) handleSyncStream(ctx context.Context, serverPubKey wgtypes.Key, sysInfo *system.Info, msgHandler func(msg *proto.SyncResponse) error, backOff backoff.BackOff) error {
 	ctx, cancelStream := context.WithCancel(ctx)
 	defer cancelStream()
 
@@ -390,6 +404,19 @@ func (c *GrpcClient) handleSyncStream(ctx context.Context, serverPubKey wgtypes.
 	c.notifyConnected()
 	c.setSyncStreamConnected()
 
+	// The stream is up, so reset the backoff. This matters for two reasons,
+	// both caused by the backoff lib not resetting its state on a successful
+	// connection:
+	//  1. Without a reset, after a connect followed by an error the next retry
+	//     starts from the accumulated (large) interval instead of retrying
+	//     promptly, delaying reconnection.
+	//  2. Worse, once the accumulated elapsed time exceeds MaxElapsedTime, the
+	//     next stream error makes NextBackOff() return Stop, so the retry loop
+	//     exits immediately. That error is then mislabeled unrecoverable and
+	//     bubbles up to trigger a full engine restart / data-plane teardown
+	//     instead of a silent reconnection.
+	backOff.Reset()
+
 	// blocking until error
 	err = c.receiveUpdatesEvents(stream, serverPubKey, msgHandler)
 	if err != nil {