refactor: extract shared read_error_body helper for upload providers

[devin-ai-integration]

Summary

Every storage-provider upload module (local, onedrive, qiniu, s3, upyun) repeated the same two lines on the HTTP error path to pull the status and body out of a failed response:

let status = response.status();
let body = response.text().await.unwrap_or_default();

This centralizes that into one shared helper so the "best-effort read, empty string on decode failure" decision lives in a single place.

New crates/cloudreve-sync/src/uploader/providers/http.rs:

pub(super) async fn read_error_body(response: Response) -> (StatusCode, String) {
    let status = response.status();
    let body = response.text().await.unwrap_or_default();
    (status, body)
}

Each provider's error branch now reads:

if !response.status().is_success() {
    let (status, body) = read_error_body(response).await;
    // ...provider-specific error formatting unchanged...
}

Applied to 9 call sites across the 5 provider files. The provider-specific error parsing/formatting (format_s3_error, QiniuError, UpyunError, OneDriveError, etc.) is left exactly as-is — only the extraction boilerplate is deduplicated, so behavior is unchanged.

Added two wiremock-backed unit tests covering the populated-body and empty-body cases.

Testing

• cargo test -p cloudreve-sync — all pass (including the 2 new tests)
• cargo clippy -p cloudreve-sync — no new warnings introduced

Link to Devin session: https://app.devin.ai/sessions/0f3275d1c9ef4100b2eb4ac3bfab22fe
Requested by: @cedev-1

[devin-ai-integration]

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