[BUG] Object storage upload keys use backslashes on Windows

Start in backend/modules/foundation/domain/file/service and run the provided Windows-focused Go test command for TestFileServiceImpl_UploadLoopFile and TestFileServiceImpl_UploadFileForServer. Trace how those methods construct object-storage keys, then verify the tests pass with platform-independent forward-slash keys, including the prefix-only space/ expectation.

📋 CheckList

• I have searched existing issues to avoid duplicates
• I am using a recently maintained version of Coze Loop
• I have provided all required information
• I understand this is a bug report and not a feature request
• I have submitted this report in English (otherwise it will not be processed)

🐛 Bug Description

UploadLoopFile and UploadFileForServer construct object-storage keys with filepath.Join. On Windows, this produces backslash-separated keys, even though object-storage keys and the existing test contract use forward slashes.

🔄 Steps to Reproduce

1. Check out current main on Windows.
2. Run:

cd backend
go test -count=1 -run 'TestFileServiceImpl_Upload(LoopFile|FileForServer)$' -v ./modules/foundation/domain/file/service

3. Observe failures in both upload methods because generated keys contain backslashes.

✅ Expected Behavior

Object-storage keys are platform-independent, for example 1234567890/testfile.txt and workspace/custom.txt.

❌ Actual Behavior

On Windows, the keys are generated as 1234567890\testfile.txt and workspace\custom.txt. Prefix-only uploads similarly fail the existing space/ expectation.

🚨 Severity

Medium - Minor functionality affected

🔧 Component

Infrastructure

💻 Environment

• OS: Windows 11
• Go: go1.26.3 windows/amd64
• Coze Loop: a2c804de6f95a209a1bd6cb6489c5a76221abd20 (current main when reproduced)

🔧 Go Environment

Relevant values: GOOS=windows, GOARCH=amd64, GOVERSION=go1.26.3.

📋 Logs

expected: "1234567890/testfile.txt"
actual  : "1234567890\\testfile.txt"

expected: "workspace/custom.txt"
actual  : "workspace\\custom.txt"

📝 Additional Context

The storage-key path should use the slash-oriented path package rather than the OS-filesystem-oriented path/filepath package. The existing tests already encode the expected portable key format.