[ISSUE #6144] 修复通过 nacos 发现下游服务时，下游服务重启后，会将旧的IP覆盖新的IP。导致通过shenyu调用下游服务报错Can not find healthy upstream url, please check your configuration!

[BraveheartStone]

修复通过 nacos 发现下游服务时，下游服务重启后，会将旧的IP覆盖新的IP。导致通过shenyu调用下游服务报错Can not find healthy upstream url, please check your configuration!

1. 缓存实例逻辑修复
2. 新增通过 selectorId 获取健康实例方法
3. weight 支持 set 方法
4. 重写 Upstream equals 方法(去除 weight 参与相等校验)

issues: #6144 #6144

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• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[Copilot]

Pull Request Overview

This PR fixes an issue where Nacos service discovery in ShenYu would incorrectly retain old IPs after downstream service restarts, causing "Can not find healthy upstream url" errors. The fix improves the upstream instance caching logic to properly handle service restarts and IP changes.

Key changes:

• Modified Upstream.equals() to exclude weight from equality checks, preventing duplicate instances with different weights
• Added setWeight() method to allow weight updates for existing upstream instances
• Refactored UpstreamCacheManager.submit() to properly handle offline instances and update weights for existing upstreams

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
Upstream.java	Made weight field mutable and excluded it from equals() comparison to support weight updates without affecting instance identity
UpstreamCheckTask.java	Added method to retrieve healthy upstream list by selector ID for cache synchronization
UpstreamCacheManager.java	Rewrote submit() logic to properly handle offline instances, update weights, and sync with healthy upstream list

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