fix: update RetryAfter handling to return null for ban policies and add custom resolver null check

docs/en/framework/infrastructure/operation-rate-limiting.md

@@ -205,7 +205,9 @@ policy.WithFixedWindow(TimeSpan.FromHours(1), maxCount: 100)
 
 ## Multi-Tenancy
 
-By default, rate limit counters are **shared across all tenants**. You can enable tenant isolation for a rule by calling `WithMultiTenancy()`:
+By default, partition keys do not include tenant information — for partition types like `PartitionByParameter`, `PartitionByCurrentUser`, `PartitionByClientIp`, etc., counters are shared across tenants unless you call `WithMultiTenancy()`. Note that `PartitionByCurrentTenant()` is inherently per-tenant since the partition key is the tenant ID itself, and `PartitionByClientIp()` is typically kept global since the same IP should share a counter regardless of tenant.
+
+You can enable tenant isolation for a rule by calling `WithMultiTenancy()`:
 
 ````csharp
 policy.AddRule(rule => rule
@@ -326,7 +328,7 @@ public override void ConfigureServices(ServiceConfigurationContext context)
 
 ### Ban Policy (maxCount: 0)
 
-Setting `maxCount` to `0` creates a ban policy that blocks all requests for the specified duration:
+Setting `maxCount` to `0` creates a ban policy that permanently denies all requests regardless of the window duration. The `RetryAfter` value will be `null` since there is no window to wait for:
 
 ````csharp
 options.AddPolicy("BlockedUser", policy =>

framework/src/Volo.Abp.OperationRateLimiting/Volo/Abp/OperationRateLimiting/Rules/FixedWindowOperationRateLimitingRule.cs

@@ -99,12 +99,24 @@ public class FixedWindowOperationRateLimitingRule : IOperationRateLimitingRule
                     $"Phone number is required for policy '{PolicyName}' (PartitionByPhoneNumber). Provide it via context.Parameter or ensure the user has a phone number."),
 
             OperationRateLimitingPartitionType.Custom =>
-                await Definition.CustomPartitionKeyResolver!(context),
+                await ResolveCustomPartitionKeyAsync(context),
 
             _ => throw new AbpException($"Unknown partition type: {Definition.PartitionType}")
         };
     }
 
+    protected virtual async Task<string> ResolveCustomPartitionKeyAsync(OperationRateLimitingContext context)
+    {
+        var key = await Definition.CustomPartitionKeyResolver!(context);
+        if (string.IsNullOrEmpty(key))
+        {
+            throw new AbpException(
+                $"Custom partition key resolver returned null or empty for policy '{PolicyName}'. " +
+                "The resolver must return a non-empty string.");
+        }
+        return key;
+    }
+
     protected virtual string BuildStoreKey(string partitionKey)
     {
         // Stable rule descriptor based on content so reordering rules does not change the key.

framework/src/Volo.Abp.OperationRateLimiting/Volo/Abp/OperationRateLimiting/Store/DistributedCacheOperationRateLimitingStore.cs

@@ -38,7 +38,7 @@ public class DistributedCacheOperationRateLimitingStore : IOperationRateLimiting
                 IsAllowed = false,
                 CurrentCount = 0,
                 MaxCount = maxCount,
-                RetryAfter = duration
+                RetryAfter = null
             };
         }
 
@@ -111,7 +111,7 @@ public class DistributedCacheOperationRateLimitingStore : IOperationRateLimiting
                 IsAllowed = false,
                 CurrentCount = 0,
                 MaxCount = maxCount,
-                RetryAfter = duration
+                RetryAfter = null
             };
         }
 

framework/test/Volo.Abp.OperationRateLimiting.Tests/Volo/Abp/OperationRateLimiting/AbpOperationRateLimitingTestModule.cs

@@ -161,6 +161,13 @@ public class AbpOperationRateLimitingTestModule : AbpModule
                       .PartitionBy(ctx => Task.FromResult($"action:{ctx.Parameter}"));
             });
 
+            // Custom resolver returning null - should throw
+            options.AddPolicy("TestCustomResolverNull", policy =>
+            {
+                policy.WithFixedWindow(TimeSpan.FromHours(1), maxCount: 2)
+                      .PartitionBy(ctx => Task.FromResult<string>(null!));
+            });
+
             // Multi-tenant: ByParameter with tenant isolation - same param, different tenants = different counters
             options.AddPolicy("TestMultiTenantByParameter", policy =>
             {

framework/test/Volo.Abp.OperationRateLimiting.Tests/Volo/Abp/OperationRateLimiting/DistributedCacheOperationRateLimitingStore_Tests.cs

@@ -118,7 +118,7 @@ public class DistributedCacheOperationRateLimitingStore_Tests : OperationRateLim
         result.IsAllowed.ShouldBeFalse();
         result.CurrentCount.ShouldBe(0);
         result.MaxCount.ShouldBe(0);
-        result.RetryAfter.ShouldNotBeNull();
+        result.RetryAfter.ShouldBeNull();
     }
 
     [Fact]
@@ -130,6 +130,6 @@ public class DistributedCacheOperationRateLimitingStore_Tests : OperationRateLim
         result.IsAllowed.ShouldBeFalse();
         result.CurrentCount.ShouldBe(0);
         result.MaxCount.ShouldBe(0);
-        result.RetryAfter.ShouldNotBeNull();
+        result.RetryAfter.ShouldBeNull();
     }
 }

framework/test/Volo.Abp.OperationRateLimiting.Tests/Volo/Abp/OperationRateLimiting/OperationRateLimitingChecker_Tests.cs

@@ -653,6 +653,7 @@ public class OperationRateLimitingChecker_Tests : OperationRateLimitingTestBase
 
         exception.Result.IsAllowed.ShouldBeFalse();
         exception.Result.MaxCount.ShouldBe(0);
+        exception.Result.RetryAfter.ShouldBeNull();
         exception.HttpStatusCode.ShouldBe(429);
     }
 
@@ -674,6 +675,7 @@ public class OperationRateLimitingChecker_Tests : OperationRateLimitingTestBase
         status.IsAllowed.ShouldBeFalse();
         status.MaxCount.ShouldBe(0);
         status.RemainingCount.ShouldBe(0);
+        status.RetryAfter.ShouldBeNull();
     }
 
     [Fact]
@@ -701,6 +703,19 @@ public class OperationRateLimitingChecker_Tests : OperationRateLimitingTestBase
         (await _checker.IsAllowedAsync("TestCustomResolver", ctx2)).ShouldBeTrue();
     }
 
+    [Fact]
+    public async Task Should_Throw_When_Custom_Resolver_Returns_Null()
+    {
+        var context = new OperationRateLimitingContext { Parameter = "test" };
+
+        var exception = await Assert.ThrowsAsync<AbpException>(async () =>
+        {
+            await _checker.CheckAsync("TestCustomResolverNull", context);
+        });
+
+        exception.Message.ShouldContain("Custom partition key resolver returned null or empty");
+    }
+
     [Fact]
     public void Should_Throw_When_Policy_Has_Duplicate_Rules()
     {