<feature>[iam2]: add ZIAM provider name constant (P1)

[zstack-robot-2]

Summary

P1 ZIAM OIDC ROPC 打通的父仓库部分：在 OAuth2PluginConstants 中新增 ZIAM_PROVIDER_NAME = "ziam" 与 LOGIN_TYPE_IAM2_ROPC = "iam2_ropc"，作为 OAuth2ClientVO.identityProvider 的判别值和登录路由 loginType，与 premium 仓库侧的 ZiamProvider / IAM2RopcLoginBackend 配套。

Resolves: ZCF-3009

Test plan

• mvn clean package 编译通过
• 172.24.246.184 casdoor 环境 PUT /v1/login loginType=iam2_ropc 冒烟 ALL PASS

sync from gitlab !9728

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 1074deb and e742258.

📒 Files selected for processing (1)

• abstraction/src/main/java/org/zstack/abstraction/sso/OAuth2PluginConstants.java

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Walkthrough

在 OAuth2PluginConstants 中新增三个公开字符串常量：ZIAM_PROVIDER_NAME、LOGIN_TYPE_IAM2_ROPC 和 ZIAM_DEFAULT_SCOPE，未修改现有常量或控制流。

Changes

Cohort / File(s)	Summary
OAuth2 常量扩展 abstraction/src/main/java/org/zstack/abstraction/sso/OAuth2PluginConstants.java	新增常量：ZIAM_PROVIDER_NAME = "ziam"、LOGIN_TYPE_IAM2_ROPC = "iam2_ropc"、ZIAM_DEFAULT_SCOPE = "openid profile email"。

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🐰 新常量跳跃来报喜，
ZIAM、ROPC、范围并齐，
小改轻添无纷争，
登录路线更明晰，
兔子鼓掌，喔耶嗨！

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题遵循 [scope]: 格式，长度59字符在72字符限制内，清晰描述了添加 ZIAM 提供商名称常量的主要变更。
Description check	✅ Passed	拉取请求描述与变更集相关，说明了添加常量的目的、涉及的文件、与其他组件的配套关系，以及测试计划。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch sync/hanyu.liang/zcf-3009@@2

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