fix(router/affinity): prevent panic caused by nil return in initialization phase

[Aetherance]

Description

在一些情况下，client 配置 affinity router 时，会因为 nil 而触发段错误

2026-02-17 21:27:55     ERROR   affinity/router.go:90       Application affinity router must set application name
2026/02/17 21:27:55 Authenticated: id=72061538416853150, timeout=4000
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xe915e9]

该 bug 只会在配置了 application 模式的 affinity router 上出现，如果配置 service 模式的 affinity router 则不会触发该 bug

配置代码:
client:

	conn, _, _ := zk.Connect([]string{"127.0.0.1:2181"}, time.Second*5)
	defer conn.Close()
	conn.Create("/dubbo/config/dubbo/router_affinity_consumer.affinity-router", []byte(`configVersion: v3.1
scope: application
key: router_affinity_consumer
enabled: true
runtime: true
affinityAware:
  key: region
  ratio: 1`), 0, zk.WorldACL(zk.PermAll))

	ins, _ := dubbo.NewInstance(
		dubbo.WithName("router_affinity_consumer"),
		dubbo.WithRegistry(registry.WithZookeeper(), registry.WithAddress("127.0.0.1:2181")),
		dubbo.WithConfigCenter(config_center.WithZookeeper(), config_center.WithAddress("127.0.0.1:2181"), config_center.WithGroup("dubbo")),
	)

	cli, _ := ins.NewClient(client.WithClientProtocolTriple())
	svc, _ := greet.NewGreetService(cli, client.WithParam("region", "shanghai"))

	ctx := context.WithValue(context.Background(),
		constant.DubboCtxKey("attachment"),
		map[string]interface{}{"region": "shanghai"},
	)

server:

	ins, _ := dubbo.NewInstance(
		dubbo.WithName("router_affinity_provider_a"),
		dubbo.WithRegistry(registry.WithZookeeper(), registry.WithAddress("127.0.0.1:2181")),
	)

	srv, _ := ins.NewServer(server.WithServerProtocol(protocol.WithTriple(), protocol.WithPort(20000)))

	handler := &AffinityGreetServer{}
	greet.RegisterGreetServiceHandler(srv, handler, server.WithParam("region", "hangzhou"))

	srv.Serve()

bug 原因是当前版本的client应用名未能被正确传递至 router chain。这导致 router chain 在 初始化 affinity router 时，由于获取到空应用名而直接返回 nil。最终造成 nil 被访问而 panic

本 pr 添加了在 router chain 读取应用名的逻辑来修复此 bug

Fixes #3203

Checklist

• I confirm the target branch is develop
• Code has passed local testing
• I have added tests that prove my fix is effective or that my feature works

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.91%. Comparing base (60d1c2a) to head (f0f2cd8).
⚠️ Report is 736 commits behind head on develop.

Files with missing lines	Patch %	Lines
cluster/router/chain/chain.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3216      +/-   ##
===========================================
+ Coverage    46.76%   47.91%   +1.15%     
===========================================
  Files          295      463     +168     
  Lines        17172    33749   +16577     
===========================================
+ Hits          8031    16172    +8141     
- Misses        8287    16269    +7982     
- Partials       854     1308     +454     

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[AlexStocks]

这里整个过程需要加上锁吧?
假设gr1 在 line 112这里获取的值为空，运行到 line114 的时候，另外一个 gr2 在 line 112 也获取为空后进入了这 if 条件，进而导致两个 gr 竞争进行 SetParam，这就不对了吧？

[Aetherance]

fixed.
plz review again

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

This PR fixes a panic in the affinity router initialization when the application name is not present in the URL by propagating it from the SubURL. The panic occurred because newApplicationAffinityRouter returns nil when the application name is empty, causing a nil pointer dereference during router chain operations.

Changes:

• Added logic to populate the application name from SubURL to the main URL in NewRouterChain
• Added a mutex to serialize access during the application name population

---

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[Copilot]

The mutex usage here is unnecessary and incorrect. The URL.SetParam method already has internal locking (paramsLock) for thread safety, as seen in common/url.go:558-564. Additionally, this global mutex serializes all NewRouterChain calls across the entire application, which could become a performance bottleneck when creating multiple router chains concurrently. Since URL modification is already thread-safe, the mutex should be removed.

[Copilot]

The PR description claims tests have been added, but there are no new test files or test functions in this change. A unit test should be added to verify that the application name is correctly propagated from SubURL when the main URL doesn't have it set, and that the fix prevents the nil panic in ApplicationAffinityRouter initialization.