perf(remotefs): send Windows file writes in 4 MiB rigrcp commands

[kke]

Uploads to Windows hosts went through winFile.CopyFrom, which used io.Copy. Upload wraps its source in an io.TeeReader, which hides any io.WriterTo, so io.Copy fell back to its 32 KiB buffer and every 32 KiB became a w command of its own: a reply to wait for before the payload, and since #480, a completion reply after it too. A 100 MiB upload made about 3,200 of those exchanges. CopyFrom now fills 4 MiB at a time, which brings that down to about 25. It does so for every source, including an *os.File, whose own WriteTo would also write 32 KiB at a time.

WriteFile had the opposite problem. bytes.Reader implements io.WriterTo, so its whole payload arrived in one Write and went out as a single w. The helper reads a command's whole payload into memory before writing it, and parses its length as a PowerShell [int]. So WriteFile held the entire file in the helper's memory and failed above 2 GiB. Write now splits a larger payload into commands of at most 4 MiB.

The idle timeout is unaffected. It is still measured per 32 KiB piece of payload handed to the pipe, not per command, so a larger command does not raise the transfer rate a slow link has to sustain.

Refs #475

[Copilot]

Copilot review overview

🔵 Needs a closer look

The test factory incorrectly captures the parent test context and must use the subtest context.

Review effort: Balanced
Findings: None

What changed in this PR

Improves Windows remote upload performance and memory usage by batching writes into 4 MiB rigrcp commands.

Changes:

• Splits large writes into bounded commands.
• Buffers CopyFrom operations in 4 MiB batches.
• Adds batching and error-handling tests.

File	Review
remotefs/​winfs_test.go	Adds coverage, but source factories must receive the subtest’s t to avoid invalid parent FailNow calls.
remotefs/​winfile.go	Implements bounded writes and buffered copying.

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[Copilot]

Copilot review overview

🟡 Changes recommended

The critical compilation failure and streaming deadlock risk must be resolved.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 2

Open (2)

• Batching CopyFrom can deadlock backpressured readers · New
• Remove duplicate writeCommandSize declaration in winfs_test.go · New

[Copilot]

Copilot review overview

🟢 Approval recommended

All reviewed changes are covered with no unresolved issues.

Review effort: Balanced
Findings: None

Resolved since last review (2)

• Remove duplicate writeCommandSize declaration in winfs_test.go
• Batching CopyFrom can deadlock backpressured readers