fix: preserve usage account attribution across retries

[yangeni]

Summary

• Keep request token usage attached to the account that produced the usage callback.
• Do not let a later retry or selector snapshot overwrite that account with another account.
• Preserve selector fallback only when no usage callback exists or the callback AuthID matches.
• Keep mismatched or unmapped usage unattributed rather than charging a different account.

Scope

• Does not change routing, retries, model pricing, quota querying, WebSocket execution IDs, or existing request logs.
• Does not rewrite historical billing records; historical reassignment requires original request-level evidence.
• No credentials or private account data are included.

Verification

• go test ./...
• go test -race ./... for the affected package
• Windows amd64 cross-compile test completed locally; CI remains the runtime verification surface.

The regression tests cover selector changes after usage, missing selector state, matching AuthID fallback, multiple successful usage records, and API-key attribution.

[jlcodes99]

已合入 main，感谢 @yangeni 提供请求用量账号归属修复！

已在当前主线完成必要的冲突适配，并通过前端测试、类型检查、翻译检查和相关 Go 回归/sidecar 构建。本地中英文 Unreleased 更新日志已补充本 PR 及你的贡献署名，后续随宿主版本发布；本次尚未发版。