fix(minimax): 补齐阈值停调与额度监控接线

调度阈值评估和渠道监控 quota 取数补上 MiniMax 分支；GetCodingPlanProvider 改为只认官方域名，避免把中转 Key 发到厂商额度端点。composite /v1/models 与 Codex 目录纳入 MiniMax；迁移改名为 237；allowlist 去掉未使用的 www.* / api.minimax.cn，abab 前缀收窄到 5/6/7。

backend/internal/service/composite_platform_test.go

@@ -179,6 +179,8 @@ func TestDetectModelPlatform(t *testing.T) {
 		{name: "minimax", model: "MiniMax-M3", platform: PlatformMiniMax, ok: true},
 		{name: "minimax prefix", model: "minimax/MiniMax-M2.5", platform: PlatformMiniMax, ok: true},
 		{name: "abab legacy", model: "abab6.5-chat", platform: PlatformMiniMax, ok: true},
+		{name: "abab7 legacy", model: "abab7-chat-preview", platform: PlatformMiniMax, ok: true},
+		{name: "abab unrelated namespace", model: "abab-other", ok: false},
 		{name: "unknown k3 alias", model: "k3-preview", ok: false},
 		{name: "unknown", model: "llama-4-maverick", ok: false},
 	}