feat(antigravity): comprehensive enhancements - model mapping, rate limiting, scheduling & ops

Key changes:
- Upgrade model mapping: Opus 4.5 → Opus 4.6-thinking with precise matching
- Unified rate limiting: scope-level → model-level with Redis snapshot sync
- Load-balanced scheduling by call count with smart retry mechanism
- Force cache billing support
- Model identity injection in prompts with leak prevention
- Thinking mode auto-handling (max_tokens/budget_tokens fix)
- Frontend: whitelist mode toggle, model mapping validation, status indicators
- Gemini session fallback with Redis Trie O(L) matching
- Ops: enhanced concurrency monitoring, account availability, retry logic
- Migration scripts: 049-051 for model mapping unification

backend/internal/service/antigravity_thinking_test.go

@@ -0,0 +1,68 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+)
+
+func TestApplyThinkingModelSuffix(t *testing.T) {
+	tests := []struct {
+		name            string
+		mappedModel     string
+		thinkingEnabled bool
+		expected        string
+	}{
+		// Thinking 未开启：保持原样
+		{
+			name:            "thinking disabled - claude-sonnet-4-5 unchanged",
+			mappedModel:     "claude-sonnet-4-5",
+			thinkingEnabled: false,
+			expected:        "claude-sonnet-4-5",
+		},
+		{
+			name:            "thinking disabled - other model unchanged",
+			mappedModel:     "claude-opus-4-6-thinking",
+			thinkingEnabled: false,
+			expected:        "claude-opus-4-6-thinking",
+		},
+
+		// Thinking 开启 + claude-sonnet-4-5：自动添加后缀
+		{
+			name:            "thinking enabled - claude-sonnet-4-5 becomes thinking version",
+			mappedModel:     "claude-sonnet-4-5",
+			thinkingEnabled: true,
+			expected:        "claude-sonnet-4-5-thinking",
+		},
+
+		// Thinking 开启 + 其他模型：保持原样
+		{
+			name:            "thinking enabled - claude-sonnet-4-5-thinking unchanged",
+			mappedModel:     "claude-sonnet-4-5-thinking",
+			thinkingEnabled: true,
+			expected:        "claude-sonnet-4-5-thinking",
+		},
+		{
+			name:            "thinking enabled - claude-opus-4-6-thinking unchanged",
+			mappedModel:     "claude-opus-4-6-thinking",
+			thinkingEnabled: true,
+			expected:        "claude-opus-4-6-thinking",
+		},
+		{
+			name:            "thinking enabled - gemini model unchanged",
+			mappedModel:     "gemini-3-flash",
+			thinkingEnabled: true,
+			expected:        "gemini-3-flash",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := applyThinkingModelSuffix(tt.mappedModel, tt.thinkingEnabled)
+			if result != tt.expected {
+				t.Errorf("applyThinkingModelSuffix(%q, %v) = %q, want %q",
+					tt.mappedModel, tt.thinkingEnabled, result, tt.expected)
+			}
+		})
+	}
+}