control/controlclient: return ctx.Err() from canceled map polls

When a map poll's context is canceled mid-read, the error that
surfaces from the response body read depends on the transport and Go
version: Go 1.26's http2 returned the context error, while Go 1.27's
returns the underlying "use of closed network connection". Normalize
the read-error paths in sendMapRequest to report ctx.Err() when the
context was canceled so callers see a stable, meaningful error
regardless of Go version.

Updates #20220

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I5898d3f452a2e61cfdf3ee015386365943c08e70