[FEAT] delete synthesizer logic

[Alanxtl]

Description

这个pr删了synthesizer相关的逻辑

1. synthesizer相关的逻辑根本就没有被使用
2. synthesizer相关的逻辑只有rest协议实现了

它最早是为应用级服务发现提供 fallback：当 consumer 通过应用实例找不到 provider exported URL metadata 时，对 REST 这类可以从实例地址推导 URL 的协议，临时合成接口级 provider URL 继续通知下游。
但现在看，它已经不符合当前架构了。当前架构的权威来源是 MetadataInfo.ServiceInfo -> instance.ToURLs()。

现在真正生效的是：

service instance -> metadata service -> MetadataInfo.Services -> instance.ToURLs(serviceInfo)

也就是用 provider 暴露出来的 ServiceInfo 重建 URL。

而 synthesizer 这边目前主要是 REST 有一个实现：根据订阅 URL 和实例地址拼：

rest://host:port/interface

但它拿不到 provider 真实导出的很多参数。更关键的是，当前代码里没有被服务发现主流程调用。

所以可以这么定性：

正常路径不该用 synthesizer。
有 metadata 时，metadata 才是权威来源。

synthesizer 最多是 fallback。
比如老 provider 没 metadata、metadata 拉取失败，但协议又能安全地从实例地址推导 URL，这时可以临时合一个。

REST synthesizer 特别鸡肋。
REST 调用真正关键的是 path/query/header/body 映射，而这些 synthesizer 推不出来。它最多能补地址，不能补完整语义。

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 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 52.53%. Comparing base (60d1c2a) to head (afe1cde).
⚠️ Report is 814 commits behind head on develop.

Files with missing lines	Patch %	Lines
cluster/router/chain/chain.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3360      +/-   ##
===========================================
+ Coverage    46.76%   52.53%   +5.76%     
===========================================
  Files          295      490     +195     
  Lines        17172    37844   +20672     
===========================================
+ Hits          8031    19880   +11849     
- Misses        8287    16358    +8071     
- Partials       854     1606     +752     

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[Copilot]

Pull request overview

This PR removes the (currently unused) Service Discovery “subscribed URL synthesizer” mechanism—including the REST synthesizer implementation—and does a small cleanup around REST config reader registration and a router-chain error message.

Changes:

• Deleted the SubscribedURLsSynthesizer interface, its factory/registry, and the REST synthesizer implementation + its unit test.
• Switched REST config reader registration to use constant.RESTProtocol instead of a local "rest" constant.
• Minor tweak to a router-chain error message punctuation (but there’s still a typo to fix: “exits” → “exists”).

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
registry/servicediscovery/synthesizer/subscribed_urls_synthesizer.go	Removes the synthesizer interface (feature deletion).
registry/servicediscovery/synthesizer/subscribed_urls_synthesizer_factory.go	Removes synthesizer registration factory (feature deletion).
registry/servicediscovery/synthesizer/rest/rest_subscribed_urls_synthesizer.go	Removes REST-specific synthesizer implementation (feature deletion).
registry/servicediscovery/synthesizer/rest/rest_subscribed_urls_synthesizer_test.go	Removes tests for the deleted REST synthesizer.
protocol/rest/config/reader/rest_config_reader.go	Uses shared constant.RESTProtocol for config reader registration.
cluster/router/chain/chain.go	Adjusts error message text (needs a small typo fix).

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[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