feat(config): validate provider existence before auxiliary fields

[Qiyuanqiii]

Description

• Reject extra_body, extra_headers, and retry_codes when a custom provider has not been configured yet.
• Preserve automatic creation for core provider fields and keep preset-provider configuration unchanged.
• Reject custom_providers.<preset-name> entries because runtime resolution always treats those names as presets, and point users to the canonical providers.* path or a different custom name.
• Recommend configuring protocol first so the remediation works for URL-based providers and URL-less custom Bedrock providers.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• make test passes locally
• Manual testing (describe below)

Additional validation:

• make check
• make coverage (90.9% total)
• ocr review --audience agent (0 findings after the review follow-up)

Checklist

• My code follows the project's coding style (go fmt, go vet)
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• I have updated the documentation accordingly (not applicable; no user-facing documentation change is needed)
• I have signed the CLA

Related Issues

Closes #1092

[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 1 selected item(s).

[Qiyuanqiii]

我认为当前错误提示仍有两个需要处理的边界问题：

1. 新测试将 custom_providers.openai.url 作为同名冲突场景的修复方式，但运行时解析会先通过 LookupProvider 判断名称；只要名称命中内置 preset，便只读取 providers.openai，不会读取 custom_providers.openai。这样提示创建的自定义配置实际上无法被选中。这里是否应该直接拒绝与 preset 同名的自定义 provider，并提示用户换名，而不是引导其创建一个不会生效的配置项？

2. 错误信息固定推荐执行 .url <endpoint>，但受支持的自定义 anthropic-bedrock provider 明确不需要 URL，应该先配置 protocol 等核心字段。是否可以将修复建议改成对所有协议都成立的表述，或者根据 provider 类型提供对应示例？

除此之外，存在性检查的位置、失败时不写入配置以及新增测试整体上没有发现其他问题。

[Qiyuanqiii]

已处理这两个边界问题，更新在 6f22039：

1. custom_providers.<preset-name> 现在会直接报错，并指向规范化后的 providers.<preset-name>.<field>，同时提示改用不同的自定义名称；测试覆盖了 OpenAI 这类大小写变体。
2. 缺失 provider 的修复示例改为先设置所有自定义 provider 都必需的 protocol，不再假设存在 URL；现有-provider 成功场景改为自定义 anthropic-bedrock，并断言 URL 保持为空。

本地已重新通过 make check、make test、make coverage（90.9%）和 ocr review --audience agent（0 findings）。

[lizhengfeng101]

LGTM