test(observability-proxy): make every bucket-cap case need the rounding

Two of the three windows divided into whole milliseconds, so they
passed with the old truncating step and guarded nothing. Each window
now leaves a sub-millisecond step remainder.

Assisted-by: LLM
Signed-off-by: ZverGuy <maximbel2003@gmail.com>

internal/observabilityproxy/logsparams_test.go

@@ -339,7 +339,7 @@ func TestBuildLogsParamsStepCannotWidenWindow(t *testing.T) {
 func TestBuildLogsParamsStepKeepsBucketCap(t *testing.T) {
 	lim := testLimits()
 	// Spans that MaxBuckets does not divide into whole milliseconds.
-	for _, start := range []string{"2026-11-03T12:00:00.0007Z", "2026-11-04T11:59:58.3333Z", "6d23h59m59s"} {
+	for _, start := range []string{"2026-11-03T12:00:00.0007Z", "2026-11-04T10:00:00.0013Z", "2026-11-04T11:00:00.0005Z"} {
 		out := build(t, "hits", url.Values{"start": {start}})
 		step, err := parseDuration(out.Get("step"))
 		if err != nil {