feat: include rotated agent logs in support bundles (#26055)

Support bundles previously captured only the active coder-agent.log, losing
history across agent restarts. Add an optional `after` filter to the agent's
/debug/logs endpoint: without it the endpoint is unchanged (active log only,
10 MiB cap); with it the response includes the active log plus rotated
coder-agent-*.log files modified after the cutoff, newest first. Support
bundles request the last 24h.

Closes #25395

agent/debuglogs_internal_test.go

@@ -0,0 +1,103 @@
+package agent
+
+import (
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+
+	"cdr.dev/slog/v3"
+	"cdr.dev/slog/v3/sloggers/slogtest"
+)
+
+func TestHandleHTTPDebugLogsWithAfterCapsResponse(t *testing.T) {
+	t.Parallel()
+
+	logDir := t.TempDir()
+	activePath := filepath.Join(logDir, "coder-agent.log")
+	f, err := os.Create(activePath)
+	require.NoError(t, err)
+	// A huge active log must not starve the rotated logs.
+	require.NoError(t, f.Truncate(debugLogsCombinedMaxBytes+1))
+	require.NoError(t, f.Close())
+
+	rotatedPath := filepath.Join(logDir, "coder-agent-2026-05-17T20-00-00.000.log")
+	require.NoError(t, os.WriteFile(rotatedPath, []byte("rotated marker\n"), 0o600))
+	rotatedModTime := time.Now().Add(-time.Minute)
+	require.NoError(t, os.Chtimes(rotatedPath, rotatedModTime, rotatedModTime))
+
+	a := &agent{
+		logger: slogtest.Make(t, &slogtest.Options{IgnoreErrors: true}).Leveled(slog.LevelDebug),
+		logDir: logDir,
+	}
+	req := httptest.NewRequest(http.MethodGet, "/debug/logs?after="+time.Now().Add(-time.Hour).UTC().Format(time.RFC3339Nano), nil)
+	res := httptest.NewRecorder()
+
+	a.HandleHTTPDebugLogs(res, req)
+
+	require.Equal(t, http.StatusOK, res.Code)
+	body := res.Body.String()
+	// Active is capped at its own limit, so the rotated log still fits.
+	require.Less(t, int64(len(body)), int64(debugLogsCombinedMaxBytes))
+	require.Contains(t, body, "coder-agent.log")
+	require.Contains(t, body, "rotated marker")
+}
+
+func TestHandleHTTPDebugLogsWithAfterOpenFailure(t *testing.T) {
+	t.Parallel()
+	if runtime.GOOS == "windows" {
+		t.Skip("unix sockets only")
+	}
+
+	logDir, err := os.MkdirTemp("/tmp", "coder-debuglogs-")
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		_ = os.RemoveAll(logDir)
+	})
+	activePath := filepath.Join(logDir, "coder-agent.log")
+	listener, err := net.Listen("unix", activePath)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		_ = listener.Close()
+	})
+
+	a := &agent{
+		logger: slogtest.Make(t, &slogtest.Options{IgnoreErrors: true}).Leveled(slog.LevelDebug),
+		logDir: logDir,
+	}
+	req := httptest.NewRequest(http.MethodGet, "/debug/logs?after="+time.Now().Add(-time.Minute).UTC().Format(time.RFC3339Nano), nil)
+	res := httptest.NewRecorder()
+
+	a.HandleHTTPDebugLogs(res, req)
+
+	require.Equal(t, http.StatusInternalServerError, res.Code)
+	require.Contains(t, res.Body.String(), "could not open log file")
+}
+
+func TestRotatedAgentLogFilesReadsLogDirLiterally(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	logDir := filepath.Join(root, "logs[abc]")
+	require.NoError(t, os.Mkdir(logDir, 0o700))
+	activePath := filepath.Join(logDir, "coder-agent.log")
+	rotatedPath := filepath.Join(logDir, "coder-agent-2026-05-18T00-00-00.000.log")
+	require.NoError(t, os.WriteFile(activePath, []byte("active log"), 0o600))
+	require.NoError(t, os.WriteFile(rotatedPath, []byte("rotated log"), 0o600))
+
+	dirRoot, err := os.OpenRoot(logDir)
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = dirRoot.Close() })
+
+	files, err := rotatedAgentLogFiles(t.Context(), slogtest.Make(t, nil), dirRoot, time.Now().Add(-time.Minute))
+
+	require.NoError(t, err)
+	require.Len(t, files, 1)
+	require.Equal(t, "coder-agent-2026-05-18T00-00-00.000.log", files[0].name)
+}