fix(stream): allow seeking beyond file size per Go convention

[Ovear]

Summary / 摘要

Go的Seek函数实现(POSIX)允许Seek到超出文件大小的位置，后续由Read返回EOF，部分类库，如sftpd-openlist依赖这个行为。

在Filezilla客户端中使用sftp下载时，当前行为会导致文件无法下载，原因是Seek返回的错误透传给客户端，而不是sftp规范中的返回EOF，导致客户端认为传输失败。

OpenList 错误日志

OpenList/internal/net/request.go:385 github.com/OpenListTeam/OpenList/v4/internal/net.(*downloader).downloadChunk() chunk_5 downloaded                           
OpenList/internal/net/request.go:629 github.com/OpenListTeam/OpenList/v4/internal/net.(*multiReadCloser).Read() read_5 finished current buffer               
sftpd-openlist/server.go:139 github.com/OpenListTeam/sftpd-openlist.ServeChannel() Read EOF, n=28642                            
sftpd-openlist/server.go:53 github.com/OpenListTeam/sftpd-openlist.ServeChannel() CR op=ssh_FXP_READ data len=22               
sftpd-openlist/server.go:62 github.com/OpenListTeam/sftpd-openlist.ServeChannel() Data 00000128000000026632000000000354000000008000 
sftpd-openlist/server.go:102 github.com/OpenListTeam/sftpd-openlist.ServeChannel() Read id=296 handle=f2 offset=55836672 length=32768 
sftpd-openlist/server.go:41 github.com/OpenListTeam/sftpd-openlist.ServeChannel() Sending error: Seek: invalid offset          
sftpd-openlist/server.go:484 github.com/OpenListTeam/sftpd-openlist.writeErrCode() Sending sftp error code ssh_FX_FAILURE       
sftpd-openlist/server.go:53 github.com/OpenListTeam/sftpd-openlist.ServeChannel() CR op=ssh_FXP_READ data len=22               
sftpd-openlist/server.go:62 github.com/OpenListTeam/sftpd-openlist.ServeChannel() Data 00000129000000026632000000000354800000008000 
sftpd-openlist/server.go:102 github.com/OpenListTeam/sftpd-openlist.ServeChannel() Read id=297 handle=f2 offset=55869440 length=32768 
sftpd-openlist/server.go:41 github.com/OpenListTeam/sftpd-openlist.ServeChannel() Sending error: Seek: invalid offset          
sftpd-openlist/server.go:484 github.com/OpenListTeam/sftpd-openlist.writeErrCode() Sending sftp error code ssh_FX_FAILURE   

FileZilla 错误日志

追踪:	Processing SSH_MSG_CHANNEL_DATA, channel=0, size=25
追踪:	Sending SSH_MSG_CHANNEL_DATA (for 27 bytes), seq=26
追踪:	sftp_base::on_read
追踪:	Processing SSH_FXP_STATUS, id=12, len=17
追踪:	Got status SSH_FX_FAILURE
错误:	Could not read from remote file: 收到错误 SSH_FX_FAILURE，没有进一步描述

Remove offset > size guard from RangeReadReadAtSeeker.Seek and DynamicReadAtSeeker.Seek. Go's io.Seeker convention allows seeking past EOF.

• This PR has breaking changes.
  / 此 PR 包含破坏性变更。
• This PR changes public API, config, storage format, or migration behavior.
  / 此 PR 修改了公开 API、配置、存储格式或迁移行为。
• This PR requires corresponding changes in related repositories.
  / 此 PR 需要关联仓库同步修改。

Testing / 测试

• go test ./...
• Manual test / 手动测试: 使用FileZilla透过sftp可以正常下载

Checklist / 检查清单

• I have read CONTRIBUTING.
  / 我已阅读 CONTRIBUTING。
• I confirm this contribution follows the repository license, contribution policy, and code of conduct.
  / 我确认此贡献符合仓库许可证、贡献规范和行为准则。
• I have formatted the changed code with gofmt, go fmt, or prettier where applicable.
  / 我已按适用情况使用 gofmt、go fmt 或 prettier 格式化变更代码。
• I have requested review from relevant maintainers or code owners where applicable.
  / 我已在适用情况下请求相关维护者或代码所有者审查。