vim25/progress: fix 'send on closed channel' panic on early upload failure

[bhagatp10]

Description

soap.Client.Upload wraps the request body in a progress.reader and defers pr.Done(err), which closes the progress channel. If the server replies before it has read the whole body (for example vCenter returning 503 mid-VMDK upload), http.Client.Do returns and Done closes the channel. net/http's writer goroutine may still be reading the body at that point. It calls reader.Read, which sends a report on the closed channel and panics with send on closed channel.

Fix:
In vim25/progress/reader.go:

• Add a sync.Mutex and a done flag to reader. Read and Done hold the lock around the channel send and close.
• After Done, Read still returns data and errors from the underlying reader, but sends no more progress reports.
• Done is idempotent, so a second call can't double-close the channel.
• pos is now guarded by the same mutex. This also removes a data race between Read (transport goroutine) and Done (caller goroutine).
• The underlying Read runs outside the lock, so a slow source never blocks Done.

Closes: #3831

How Has This Been Tested?

• Added new tests TestReaderReadAfterDone, TestReaderConcurrentReadDone, TestReaderReadError and TestUploadEarlyErrorResponse to cover different scenarios.
• Ran go test -race -count=5 ./vim25/progress ./vim25/soap and go test -race ./vim25/... ./object ./nfc to run these tests.