[e2e][flake] Prevent Windows flare archive collisions (#54911)

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### What does this PR do?

- Adds a process-local atomic counter suffix to flare archive filenames.
- Includes the process ID with the counter to keep the suffix readable and avoid same-second collisions with another local process.
- Keeps the existing timestamp and log-level filename context.
- Adds a deterministic regression test that verifies repeated archive names generated for the same timestamp do not collide.

### Motivation

Windows fails `os.Rename` when the destination archive already exists. `TestAgentTaskFlareSourceAndTags` creates several flare archives inside the same test process, with second-precision filenames. Once every ~2k runs, subtests created flares exactly at the same second in time, colliding on the file name. 

A process-local atomic counter directly addresses those rapid same-process creations.

Jira: FLREM-150

### Describe how you validated your changes

- Test passes on the CI
Will keep monitoring for further failures.

### Additional Notes

None.

---

PR with help of Bits - [View session in Datadog](https://app.datadoghq.com/code/c5d0b2d1-7ec1-45c4-a402-380c09336a86)

Co-authored-by: datadog-bits <263423550+datadog-bits@users.noreply.github.com>
Co-authored-by: paola.ducolin <paola.ducolin@datadoghq.com>

Author: pducolin

comp/core/flare/helpers/builder.go

@@ -16,6 +16,7 @@ import (
 	"regexp"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/DataDog/datadog-agent/comp/core/flare/types"
@@ -31,6 +32,8 @@ const (
 	filePerm = 0644
 )
 
+var archiveNameCounter atomic.Uint64
+
 func newBuilder(root string, hostname string, localFlare bool, flareArgs types.FlareArgs) (*builder, error) {
 	fb := &builder{
 		tmpDir:           root,
@@ -144,7 +147,10 @@ type builder struct {
 }
 
 func getArchiveName() string {
-	t := time.Now().UTC()
+	return getArchiveNameForTime(time.Now().UTC(), newArchiveNameID())
+}
+
+func getArchiveNameForTime(t time.Time, uniqueSuffix string) string {
 	timeString := strings.ReplaceAll(t.Format(time.RFC3339), ":", "-")
 
 	logLevel, err := log.GetLogLevel()
@@ -153,7 +159,11 @@ func getArchiveName() string {
 		logLevelString = "-" + logLevel.String()
 	}
 
-	return fmt.Sprintf("datadog-agent-%s%s.zip", timeString, logLevelString)
+	return fmt.Sprintf("datadog-agent-%s-%s%s.zip", timeString, uniqueSuffix, logLevelString)
+}
+
+func newArchiveNameID() string {
+	return fmt.Sprintf("%d-%d", os.Getpid(), archiveNameCounter.Add(1))
 }
 
 func (fb *builder) Save() (string, error) {

comp/core/flare/helpers/builder_test.go

@@ -122,6 +122,19 @@ func TestSave(t *testing.T) {
 	}
 }
 
+func TestGetArchiveNameIsUniqueWithinSameSecond(t *testing.T) {
+	timestamp := time.Date(2026, time.August, 14, 12, 0, 0, 0, time.UTC)
+	names := make(map[string]struct{})
+
+	for range 100 {
+		name := getArchiveNameForTime(timestamp, newArchiveNameID())
+		if _, found := names[name]; found {
+			t.Fatalf("archive name %q was generated more than once", name)
+		}
+		names[name] = struct{}{}
+	}
+}
+
 func TestAddFileFromFunc(t *testing.T) {
 	fb := getNewBuilder(t)
 	defer fb.clean()