fix(agent): propagate file upload read errors

[felix-windsor]

Summary

• propagate non-EOF errors returned by io.ReadFull during file uploads
• preserve the existing EOF and final-partial-chunk behavior
• add focused tests for full, partial, empty, and failed reads

Problem

The upload loop stores the result of io.ReadFull in rerr, but checks the unrelated outer err variable. A disk or network-filesystem read error is therefore ignored, and the loop may send an incomplete chunk as if the read succeeded.

The helper introduced here classifies successful/full reads, normal completion (io.EOF and io.ErrUnexpectedEOF), and actual read failures explicitly. The caller returns before client.Send when a real read failure occurs.

Validation

• go test ./transport/file_ext.go ./transport/file_ext_test.go
• GOOS=linux GOARCH=amd64 go test -c -o /tmp/elkeid-transport.test ./transport
• GOOS=linux GOARCH=amd64 go vet ./transport
• go vet ./transport/file_ext.go ./transport/file_ext_test.go

The package-wide test executable is cross-compiled because the agent only provides plugin.Load and Plugin.Shutdown implementations for Linux.

Fixes #748