Add unit tests for pkranges link URL encoding fix

Add three tests in partition_key_range_cache.rs that verify:
- item_by_rid() preserves '=' in RIDs like "pLLZAIuPigw=" (no %3D)
- item() incorrectly URL-encodes '=' to '%3D' (documenting the bug)
- item_by_rid() also preserves '+' and '/' in base64 RIDs

Co-authored-by: tvaron3 <70857381+tvaron3@users.noreply.github.com>
Agent-Logs-Url: https://github.com/Azure/azure-sdk-for-rust/sessions/f515482f-d87e-4105-9421-6ff7dac1508b

Author: Copilot

sdk/cosmos/azure_data_cosmos/src/routing/partition_key_range_cache.rs

@@ -684,4 +684,61 @@ mod tests {
         assert!(range.target_throughput.is_some());
         assert_eq!(range.target_throughput.unwrap(), 1000.0);
     }
+
+    // Tests verifying that the pkranges resource link uses item_by_rid() so that
+    // collection RIDs (which are base64-encoded and can contain '=', '+', '/') are
+    // not URL-percent-encoded. Using item() would encode '=' to '%3D', causing 404s.
+
+    #[test]
+    fn pkranges_link_rid_with_equals_is_not_encoded() {
+        // RIDs like "pLLZAIuPigw=" contain '=' which item() would encode to '%3D'.
+        // item_by_rid() must preserve it as-is.
+        let collection_rid = "pLLZAIuPigw=";
+        let database_link = ResourceLink::root(ResourceType::Databases).item("perfdb");
+        let pk_range_link = database_link
+            .feed(ResourceType::Containers)
+            .item_by_rid(collection_rid)
+            .feed(ResourceType::PartitionKeyRanges);
+
+        // Correct: '=' preserved, not encoded to '%3D'
+        assert_eq!(
+            "dbs/perfdb/colls/pLLZAIuPigw=/pkranges",
+            pk_range_link.path()
+        );
+    }
+
+    #[test]
+    fn pkranges_link_item_encodes_equals_incorrectly() {
+        // Demonstrates the bug: item() URL-encodes '=' to '%3D', producing a path
+        // that Cosmos DB cannot find (404).
+        let collection_rid = "pLLZAIuPigw=";
+        let database_link = ResourceLink::root(ResourceType::Databases).item("perfdb");
+        let pk_range_link_wrong = database_link
+            .feed(ResourceType::Containers)
+            .item(collection_rid)
+            .feed(ResourceType::PartitionKeyRanges);
+
+        // Wrong: '=' is encoded to '%3D', causing 404 from Cosmos DB
+        assert!(
+            pk_range_link_wrong.path().contains("%3D"),
+            "item() should URL-encode '=' to '%3D'"
+        );
+        assert_eq!(
+            "dbs/perfdb/colls/pLLZAIuPigw%3D/pkranges",
+            pk_range_link_wrong.path()
+        );
+    }
+
+    #[test]
+    fn pkranges_link_rid_with_plus_is_not_encoded() {
+        // RIDs may also contain '+' (base64 char). item_by_rid() must preserve it.
+        let collection_rid = "AB+CD/EF==";
+        let database_link = ResourceLink::root(ResourceType::Databases).item("mydb");
+        let pk_range_link = database_link
+            .feed(ResourceType::Containers)
+            .item_by_rid(collection_rid)
+            .feed(ResourceType::PartitionKeyRanges);
+
+        assert_eq!("dbs/mydb/colls/AB+CD/EF==/pkranges", pk_range_link.path());
+    }
 }