feat: implement credential-based round-robin for gemini-cli virtual auths

Changes the RoundRobinSelector to use two-level round-robin when
gemini-cli virtual auths are detected (via gemini_virtual_parent attr):
- Level 1: cycle across credential groups (parent accounts)
- Level 2: cycle within each group's project auths

Credentials start from a random offset (rand.IntN) for fair distribution.
Non-virtual auths and single-credential scenarios fall back to flat RR.

Adds 3 test cases covering multi-credential grouping, single-parent
fallback, and mixed virtual/non-virtual fallback.

sdk/cliproxy/auth/selector.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"math"
+	"math/rand/v2"
 	"net/http"
 	"sort"
 	"strconv"
@@ -248,6 +249,9 @@ func getAvailableAuths(auths []*Auth, provider, model string, now time.Time) ([]
 }
 
 // Pick selects the next available auth for the provider in a round-robin manner.
+// For gemini-cli virtual auths (identified by the gemini_virtual_parent attribute),
+// a two-level round-robin is used: first cycling across credential groups (parent
+// accounts), then cycling within each group's project auths.
 func (s *RoundRobinSelector) Pick(ctx context.Context, provider, model string, opts cliproxyexecutor.Options, auths []*Auth) (*Auth, error) {
 	_ = opts
 	now := time.Now()
@@ -265,21 +269,87 @@ func (s *RoundRobinSelector) Pick(ctx context.Context, provider, model string, o
 	if limit <= 0 {
 		limit = 4096
 	}
-	if _, ok := s.cursors[key]; !ok && len(s.cursors) >= limit {
-		s.cursors = make(map[string]int)
-	}
-	index := s.cursors[key]
 
+	// Check if any available auth has gemini_virtual_parent attribute,
+	// indicating gemini-cli virtual auths that should use credential-level polling.
+	groups, parentOrder := groupByVirtualParent(available)
+	if len(parentOrder) > 1 {
+		// Two-level round-robin: first select a credential group, then pick within it.
+		groupKey := key + "::group"
+		s.ensureCursorKey(groupKey, limit)
+		if _, exists := s.cursors[groupKey]; !exists {
+			// Seed with a random initial offset so the starting credential is randomized.
+			s.cursors[groupKey] = rand.IntN(len(parentOrder))
+		}
+		groupIndex := s.cursors[groupKey]
+		if groupIndex >= 2_147_483_640 {
+			groupIndex = 0
+		}
+		s.cursors[groupKey] = groupIndex + 1
+
+		selectedParent := parentOrder[groupIndex%len(parentOrder)]
+		group := groups[selectedParent]
+
+		// Second level: round-robin within the selected credential group.
+		innerKey := key + "::cred:" + selectedParent
+		s.ensureCursorKey(innerKey, limit)
+		innerIndex := s.cursors[innerKey]
+		if innerIndex >= 2_147_483_640 {
+			innerIndex = 0
+		}
+		s.cursors[innerKey] = innerIndex + 1
+		s.mu.Unlock()
+		return group[innerIndex%len(group)], nil
+	}
+
+	// Flat round-robin for non-grouped auths (original behavior).
+	s.ensureCursorKey(key, limit)
+	index := s.cursors[key]
 	if index >= 2_147_483_640 {
 		index = 0
 	}
-
 	s.cursors[key] = index + 1
 	s.mu.Unlock()
-	// log.Debugf("available: %d, index: %d, key: %d", len(available), index, index%len(available))
 	return available[index%len(available)], nil
 }
 
+// ensureCursorKey ensures the cursor map has capacity for the given key.
+// Must be called with s.mu held.
+func (s *RoundRobinSelector) ensureCursorKey(key string, limit int) {
+	if _, ok := s.cursors[key]; !ok && len(s.cursors) >= limit {
+		s.cursors = make(map[string]int)
+	}
+}
+
+// groupByVirtualParent groups auths by their gemini_virtual_parent attribute.
+// Returns a map of parentID -> auths and a sorted slice of parent IDs for stable iteration.
+// Only auths with a non-empty gemini_virtual_parent are grouped; if any auth lacks
+// this attribute, nil/nil is returned so the caller falls back to flat round-robin.
+func groupByVirtualParent(auths []*Auth) (map[string][]*Auth, []string) {
+	if len(auths) == 0 {
+		return nil, nil
+	}
+	groups := make(map[string][]*Auth)
+	for _, a := range auths {
+		parent := ""
+		if a.Attributes != nil {
+			parent = strings.TrimSpace(a.Attributes["gemini_virtual_parent"])
+		}
+		if parent == "" {
+			// Non-virtual auth present; fall back to flat round-robin.
+			return nil, nil
+		}
+		groups[parent] = append(groups[parent], a)
+	}
+	// Collect parent IDs in sorted order for stable cursor indexing.
+	parentOrder := make([]string, 0, len(groups))
+	for p := range groups {
+		parentOrder = append(parentOrder, p)
+	}
+	sort.Strings(parentOrder)
+	return groups, parentOrder
+}
+
 // Pick selects the first available auth for the provider in a deterministic manner.
 func (s *FillFirstSelector) Pick(ctx context.Context, provider, model string, opts cliproxyexecutor.Options, auths []*Auth) (*Auth, error) {
 	_ = opts