fix: resolve data race in uptoken.NewSigner

[xgopilot]

Requested by @zhangzqs

Summary

Fixed a critical thread-safety issue in the storagev2/uptoken package where concurrent calls to GetUpToken() caused data races during parallel file uploads.

Problem

The data race detector identified concurrent read/write operations on signer.upToken field:

• Read at line 89: if signer.upToken != ""
• Write at line 100: signer.upToken = credentials.SignWithData(putPolicyJson)

When multiple goroutines called GetUpToken() simultaneously (e.g., during UploadDirectory operations), they would:

1. Both read the empty upToken field
2. Both proceed to generate and write the token
3. Create a data race condition

Solution

Implemented thread-safe token initialization using sync.Once:

• Added onceUpToken sync.Once field to the signer struct
• Refactored onceGetUpToken() to use sync.Once.Do() for initialization
• This ensures token generation happens exactly once, even under concurrent access

The fix follows the same pattern already used in onceGetCredentials() method, maintaining consistency with the existing codebase.

Testing

The original issue was detected during concurrent upload operations in UploadDirectory. This fix ensures that:

• Token generation is thread-safe
• Multiple concurrent calls return the same cached token
• No performance degradation (tokens are still cached after first generation)

Fixes #169

Generated with codeagent

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