fix(search): clamp auto histogram interval to 1s for sub-10s ranges (#13916)

## What

For time ranges narrower than 10 seconds, the auto-generated histogram
interval now drops to `1 second` instead of the previous `10 second`
floor. Implemented as a new `s` unit in the `intervals!` ladder in
`generate_histogram_interval` plus a `(s, 10, "10 second")` tier, with
the fall-through returning `1 second`. All tiers from 10s upward are
unchanged, and intervals written explicitly in user SQL are untouched.

## Why

Fixes #13896. With the 10s floor, a 1-second query range got a bucket
wider than the whole range. The scan-start snap to the bucket boundary
(`histogram_bucket_start`, introduced by #12312 to keep the first chart
bucket complete) then pulled in up to 10s of data from before the
selected range, so histogram bucket counts dwarfed the log search total
— the UI event counter and pagination are derived from the bucket sum,
producing e.g. "1 to 50 out of 195 events" with only 8 real rows and
empty trailing pages.

With 1s buckets, whole-second range starts sit exactly on the bucket
grid, so the snap no longer widens the scan and the bucket sum matches
the log search total.

## Verification

- Unit tests added for the 1s/10s boundary; full `search` crate suite
passes.
- E2E reproduction (single node, 20 events/sec ingested, 1s mid-bucket
range `[hh:mm:s4, hh:mm:s5)`):
- before: log search `total: 20`, histogram bucket sum **100** (follower
plan showed `FilterExec: _timestamp >= …s0` — start snapped back 4s)
- after: log search `total: 20`, histogram bucket sum **20**,
`FilterExec` covers exactly the requested 1s range

## Notes for reviewers

- The snap behavior from #12312 itself is intentionally left untouched;
this fix only ensures the derived bucket is never wider than the range.
- The clamp is deliberately **not** added to
`validate_and_adjust_histogram_interval`: that path validates
caller-supplied intervals (explicit SQL intervals, and the per-partition
re-validation of the streaming pre-computed interval), where
range-dependence would make partitions disagree — see
`test_preset_interval_is_consistent_across_partitions`.
- Millisecond-precision range starts (relative time pickers) can still
be snapped back by up to 1s; that residue is bounded by one 1s bucket
versus up to 10s before.

src/search/src/sql/visitor/histogram_interval.rs

@@ -87,6 +87,7 @@ macro_rules! intervals {
 
     (@unit h) => { chrono::Duration::hours };
     (@unit m) => { chrono::Duration::minutes };
+    (@unit s) => { chrono::Duration::seconds };
 }
 
 pub fn generate_histogram_interval(time_range: (i64, i64)) -> &'static str {
@@ -106,7 +107,7 @@ pub fn generate_histogram_interval(time_range: (i64, i64)) -> &'static str {
         (h, 2, "1 minute"),
         (h, 1, "30 second"),
         (m, 30, "15 second"),
-        (m, 15, "10 second"),
+        (s, 10, "10 second"),
     ];
 
     for (time, interval) in INTERVALS.iter() {
@@ -114,7 +115,12 @@ pub fn generate_histogram_interval(time_range: (i64, i64)) -> &'static str {
             return interval;
         }
     }
-    "10 second"
+    // Ranges narrower than one 10s bucket drop to 1s buckets: a bucket wider
+    // than the whole range makes the scan-start snap (histogram_bucket_start)
+    // pull in up to 10s of data from before the range, so bucket counts dwarf
+    // the range total (#13896). With 1s buckets the snap widens the scan by
+    // less than a second (zero for whole-second range starts).
+    "1 second"
 }
 
 pub fn convert_histogram_interval_to_seconds(interval: &str) -> Result<i64, Error> {
@@ -526,12 +532,26 @@ mod tests {
         assert_eq!(generate_histogram_interval((0, 24 * 31 * hour)), "12 hour");
         // > 24*28h but <= 24*30h → "6 hour"
         assert_eq!(generate_histogram_interval((0, 24 * 29 * hour)), "6 hour");
-        // duration < 15 min → falls through all checks → "10 second"
+        // 10s <= duration < 30 min → "10 second"
         assert_eq!(generate_histogram_interval((0, 10 * minute)), "10 second");
         // duration >= 30 min → "15 second"
         assert_eq!(generate_histogram_interval((0, 31 * minute)), "15 second");
     }
 
+    #[test]
+    fn test_generate_histogram_interval_narrow_range() {
+        let second = 1_000_000_i64;
+        // a range narrower than the 10s floor drops to 1s buckets so the
+        // scan-start snap cannot widen the query past the range (#13896)
+        assert_eq!(generate_histogram_interval((0, second)), "1 second");
+        assert_eq!(
+            generate_histogram_interval((0, 10 * second - 1)),
+            "1 second"
+        );
+        // at exactly 10s the historical floor is kept
+        assert_eq!(generate_histogram_interval((0, 10 * second)), "10 second");
+    }
+
     #[test]
     fn test_convert_histogram_interval_abbreviations() {
         // Test single-letter and short abbreviations