Don't start backoff timer if it would exceed context deadline

[c-tonneslan]

If the calculated backoff wait would push past the context deadline, we're just going to block until the deadline fires and get context.DeadlineExceeded anyway. This change checks for that upfront and returns immediately instead of sleeping unnecessarily.

The fix is a single deadline check before time.NewTimer:

if deadline, ok := req.Context().Deadline(); ok && time.Now().Add(wait).After(deadline) {
    c.HTTPClient.CloseIdleConnections()
    return nil, req.Context().Err()
}

This makes callers with tight deadlines get the error back faster, without the full backoff sleep eating into their remaining budget.

Existing tests pass (go test ./...).

Fixes #272

[hashicorp-cla-app]

All committers have signed the CLA.