[Open Codesign] Enhance provider error normalization with recovery actions

# [Feature]: 增强 Provider 错误归一化与恢复提示 / Improve provider error normalization and recovery actions ## Problem / pain point / 问题 现在已经有 provider error normalization，但恢复指引还不够稳定。不同 gateway 的失败方式很多： - auth failure - endpoint shape 不兼容 - partial compatibility - upstream “not implemented” - request shape rejection 用户很多时候不只是需要一个 normalized error string，而是需要“下一步怎么修”。 We already normalize provider errors, but users often need the next action, not only the normalized error details. ## Proposed solution / 方案 基于现有 normalization 增加 recovery categories 和 actionable hints，例如： - base URL 需要补 `/v1` - 从 `responses` 切到 `chat/completions` - 对当前 model 关闭 reasoning - 改成 manual model entry - 重新导入 provider credentials - 补充 gateway-specific headers Build on the current normalization layer rather than replacing it. ## Alternatives considered / 备选方案 - 只输出技术细节，不做恢复动作提示 - 对用户和排障都不够友好 ## Scope / 范围 Provider / model ## Acceptance criteria / 验收标准 - normalized error 可映射到 recovery category - 多类已知兼容问题能给出 ta