fix(client): preserve typed *HTTPStatusError when ctx is cancelled at Close

[worstell]

writeCloser.Close discards the value from <-wc.done whenever wc.ctx.Err() is non-nil, returning only create operation cancelled: context canceled. This masks an authoritative server response (e.g. 403 Forbidden) when the response arrives before the upload pipe is torn down — exactly what happens when the server denies a write mid-stream.

The fix:

• Drain the upload pipe (CloseWithError on cancel, Close otherwise) so the request goroutine can finish.
• Always wait on <-wc.done so we can inspect the server's response.
• Prefer the typed *HTTPStatusError when present; fall through to the local ctx / pipe-close error only when the server didn't send a typed status.

Restores the contract callers depend on: errors.As(err, &*HTTPStatusError{}) works after Save even when the response races a context cancellation.

While the cancel path is being reshaped, two adjacent issues are fixed in the same change so we don't carry them forward:

• Abort(cause) now propagates the caller's cause. Close reads context.Cause(ctx) instead of ctx.Err(), so the pipe is closed with — and the wrapped error contains — the cause passed to Abort rather than a generic context.Canceled. When the parent context cancels independently, Cause falls back to ctx.Err so behaviour matches the previous code in that path.
• Close is now idempotent under concurrent calls. The previous closed bool guard was racy and, more importantly, the second caller would deadlock on the drained <-wc.done channel — a real risk under patterns like a deferred Close racing an explicit Abort. Replaced with sync.Once so the close-and-wait runs at most once and the second caller returns the cached error immediately.

Note: callers that match the literal string create operation cancelled to detect a cancelled-with-server-response Close will now see request failed: ... instead. This is intentional — the documented contract is errors.As against *HTTPStatusError.

Tests

• TestCreateClosePreservesStatusErrorOnCancelledCtx — controllable RoundTripper deterministically reproduces the masked-403 race (response delivered, then ctx cancelled, then Close). Without the fix the test fails with the exact error string seen in production.
• TestCreateCloseIdempotent — second Close call must return without blocking on the drained done channel.