chore: extract testutil.FakeSink for slog test assertions (#23208)

Follow-up to [review comment on
#23025](https://github.com/coder/coder/pull/23025#discussion_r2930309487)
from @mafredri.

Extracts the repeated `logSink` / `fakeSink` test pattern into a shared
`testutil.FakeSink` and migrates all existing call sites.

> 🤖 This PR was created with the help of Coder Agents, and will be
reviewed by my human. 🧑‍💻

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

agent/boundary_logs_test.go

@@ -6,7 +6,6 @@ import (
 	"context"
 	"net"
 	"path/filepath"
-	"sync"
 	"testing"
 
 	"github.com/google/uuid"
@@ -23,26 +22,6 @@ import (
 	"github.com/coder/coder/v2/testutil"
 )
 
-// logSink captures structured log entries for testing.
-type logSink struct {
-	mu      sync.Mutex
-	entries []slog.SinkEntry
-}
-
-func (s *logSink) LogEntry(_ context.Context, e slog.SinkEntry) {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-	s.entries = append(s.entries, e)
-}
-
-func (*logSink) Sync() {}
-
-func (s *logSink) getEntries() []slog.SinkEntry {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-	return append([]slog.SinkEntry{}, s.entries...)
-}
-
 // getField returns the value of a field by name from a slog.Map.
 func getField(fields slog.Map, name string) interface{} {
 	for _, f := range fields {
@@ -76,8 +55,8 @@ func TestBoundaryLogs_EndToEnd(t *testing.T) {
 	require.NoError(t, err)
 	t.Cleanup(func() { require.NoError(t, srv.Close()) })
 
-	sink := &logSink{}
-	logger := slog.Make(sink)
+	sink := testutil.NewFakeSink(t)
+	logger := sink.Logger(slog.LevelInfo)
 	workspaceID := uuid.New()
 	templateID := uuid.New()
 	templateVersionID := uuid.New()
@@ -118,10 +97,10 @@ func TestBoundaryLogs_EndToEnd(t *testing.T) {
 	sendBoundaryLogsRequest(t, conn, req)
 
 	require.Eventually(t, func() bool {
-		return len(sink.getEntries()) >= 1
+		return len(sink.Entries()) >= 1
 	}, testutil.WaitShort, testutil.IntervalFast)
 
-	entries := sink.getEntries()
+	entries := sink.Entries()
 	require.Len(t, entries, 1)
 	entry := entries[0]
 	require.Equal(t, slog.LevelInfo, entry.Level)
@@ -152,10 +131,10 @@ func TestBoundaryLogs_EndToEnd(t *testing.T) {
 	sendBoundaryLogsRequest(t, conn, req2)
 
 	require.Eventually(t, func() bool {
-		return len(sink.getEntries()) >= 2
+		return len(sink.Entries()) >= 2
 	}, testutil.WaitShort, testutil.IntervalFast)
 
-	entries = sink.getEntries()
+	entries = sink.Entries()
 	entry = entries[1]
 	require.Len(t, entries, 2)
 	require.Equal(t, slog.LevelInfo, entry.Level)