fix: use integer truncation in FormatAge to prevent rounding past display boundaries

Math.Round with C# default banker's rounding (MidpointRounding.ToEven) can
produce counterintuitive results at display-threshold boundaries:

- 59.5 minutes -> Math.Round(59.5) = 60 -> displayed as '60m ago'
  instead of '59m ago' (or the correct transition to '1h ago')
- 47.5 hours -> Math.Round(47.5) = 48 -> displayed as '48h ago'
  instead of '47h ago' (near the 48h/days boundary)

Using integer truncation ((int)delta.TotalX) matches the idiomatic
convention for age display: show the floor of the elapsed time, which
is consistent, predictable, and never exceeds the guard condition.

Adds three regression tests covering:
- 59.5-minute boundary (was '60m ago', now '59m ago')
- 47.5-hour boundary (was '48h ago', now '47h ago')
- Exactly 60 seconds (correctly '1m ago')

Test status: Shared.Tests 589 passed, 20 skipped; Tray.Tests 122 passed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/OpenClaw.Shared/Models.cs

@@ -441,9 +441,9 @@ internal static class ModelFormatting
     {
         var delta = DateTime.UtcNow - timestampUtc;
         if (delta.TotalSeconds < 60) return "just now";
-        if (delta.TotalMinutes < 60) return $"{(int)Math.Round(delta.TotalMinutes)}m ago";
-        if (delta.TotalHours < 48) return $"{(int)Math.Round(delta.TotalHours)}h ago";
-        return $"{(int)Math.Round(delta.TotalDays)}d ago";
+        if (delta.TotalMinutes < 60) return $"{(int)delta.TotalMinutes}m ago";
+        if (delta.TotalHours < 48) return $"{(int)delta.TotalHours}h ago";
+        return $"{(int)delta.TotalDays}d ago";
     }
 
     /// <summary>

tests/OpenClaw.Shared.Tests/ModelsTests.cs

@@ -762,6 +762,31 @@ public class SessionInfoAgeTextTests
         };
         Assert.Equal("10m ago", session.AgeText);
     }
+
+    [Fact]
+    public void AgeText_NearMinuteBoundary_DoesNotRoundUpTo60m()
+    {
+        // 59.5 minutes: Math.Round would produce 60 with banker's rounding;
+        // truncation correctly yields 59m ago.
+        var session = new SessionInfo { UpdatedAt = DateTime.UtcNow.AddSeconds(-3570) }; // 59.5 min
+        Assert.Equal("59m ago", session.AgeText);
+    }
+
+    [Fact]
+    public void AgeText_NearHourBoundary_DoesNotRoundUpTo48h()
+    {
+        // 47.5 hours: Math.Round would produce 48 with banker's rounding;
+        // truncation correctly yields 47h ago.
+        var session = new SessionInfo { UpdatedAt = DateTime.UtcNow.AddSeconds(-(int)(47.5 * 3600)) };
+        Assert.Equal("47h ago", session.AgeText);
+    }
+
+    [Fact]
+    public void AgeText_ExactlyOneMinute_ShowsMinutesAgo()
+    {
+        var session = new SessionInfo { UpdatedAt = DateTime.UtcNow.AddSeconds(-60) };
+        Assert.Equal("1m ago", session.AgeText);
+    }
 }
 
 public class SessionInfoRichDisplayTextTests