resolver: a query cut short by the caller's context records no reason (closes #298)

When the caller's context ends, the next query returns
ErrContextCanceled before anything is sent. querySingleType recorded
that as a network error, which outranks a timeout, so a nameserver that
had only timed out could be reported as "network error: context
canceled". It now records no reason for it.

Model: opus-5-5

TODO.md

@@ -19,6 +19,9 @@ milestone: https://git.eeqj.de/sneak/dnswatcher/milestone/7
 
 # Completed Steps
 
+- 2026-10-15: a query cut short by its caller's context records no reason about
+  the nameserver, so one whose only failures were timeouts is `timeout`, not a
+  network error (closes #298).
 - 2026-10-15: the resolver no longer says its query timeout grows on each try;
   every try waits the same 2 seconds, and the unused `timeoutMultiplier` is gone
   (closes #296).

internal/resolver/iterative.go

@@ -863,6 +863,10 @@ func (r *Resolver) querySingleType(
 			if err != nil {
 				err = fmt.Errorf("query %s @%s: %w", hostname, nsIP, err)
 			}
+		case errors.Is(err, ErrContextCanceled):
+			// The caller's context ended and cut the query short. That
+			// says nothing about the nameserver, so no reason is
+			// recorded.
 		case isTimeout(err):
 			state.gotTimeout = true
 		case errors.Is(err, ErrRefused):

internal/resolver/iterative_internal_test.go

@@ -1,6 +1,8 @@
 package resolver
 
 import (
+	"context"
+	"log/slog"
 	"strconv"
 	"syscall"
 	"testing"
@@ -132,3 +134,33 @@ func TestReadReply(t *testing.T) {
 		})
 	}
 }
+
+// TestQuerySingleType_ContextEnded asks a nameserver about one record
+// type with a context that has already ended, so no query is sent. A
+// query the context cut short says nothing about the nameserver and
+// records no reason: a nameserver whose earlier query timed out is
+// still reported as timed out, not as a network error.
+func TestQuerySingleType_ContextEnded(t *testing.T) {
+	t.Parallel()
+
+	r := NewFromLogger(slog.New(slog.DiscardHandler))
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	resp := &NameserverResponse{Records: map[string][]string{}, Status: StatusOK}
+
+	// The query for an earlier record type timed out.
+	state := queryState{gotTimeout: true}
+
+	_, err := r.querySingleType(
+		ctx, []string{"192.0.2.1"}, "example.com.", dns.TypeA, resp, &state,
+	)
+	require.ErrorIs(t, err, ErrContextCanceled)
+	assert.Equal(t, queryState{gotTimeout: true}, state)
+
+	classifyResponse(resp, state)
+
+	assert.Equal(t, StatusTimeout, resp.Status)
+	assert.Equal(t, "all queries timed out", resp.Error)
+}