fix(integrations/unftp-sbe): avoid copy flush amplification

[Xuanwo]

Which issue does this PR close?

N/A (related to discussion #7200).

Rationale for this change

unftp-sbe currently uses tokio::io::copy in put. Under slow uploads with small packets, copy can hit pending-read paths that flush the writer frequently, which interacts poorly with buffered multipart writers and can amplify memory usage.

What changes are included in this PR?

• Replace tokio::io::copy with an explicit read/write loop in integrations/unftp-sbe/src/lib.rs.
• Keep transfer behavior simple and backpressure-driven via read + write_all.
• Keep existing shutdown and error handling behavior unchanged.

Validation:

• cargo check --lib (in integrations/unftp-sbe)
• cargo clippy --lib -- -D warnings (in integrations/unftp-sbe)
• cargo test --lib (in integrations/unftp-sbe)

Are there any user-facing changes?

No API changes. This adjusts internal upload transfer behavior to be more stable under slow/small-packet upload patterns.

AI Usage Statement

This PR was prepared with OpenAI Codex (GPT-5) for code editing and review support.