Fix MIN/MAX aggregation for empty partitions and add regression test (#36177)

### Packages impacted by this PR
@azure/cosmos

### Issues associated with this PR

### Describe the problem that is addressed by this PR
Fixed MIN/MAX aggregate queries randomly returning incorrect results in
multi-partition collections. Empty partitions (count:0) were incorrectly
overwriting valid aggregate values with undefined, causing MIN or MAX
values to be missing from query results.

### What are the possible designs available to address the problem? If
there are more than one possible design, why was the one in this PR
chosen?


### Are there test cases added in this PR? _(If not, why?)_
yes

### Provide a list of related PRs _(if any)_


### Command used to generate this PR:**_(Applicable only to SDK release
request PRs)_

### Checklists
- [ ] Added impacted package name to the issue description
- [ ] Does this PR needs any fixes in the SDK Generator?** _(If so,
create an Issue in the
[Autorest/typescript](https://github.com/Azure/autorest.typescript)
repository and link it here)_
- [ ] Added a changelog (if necessary)

---------

Co-authored-by: Manik Khandelwal <[email protected]>

Author: topshot99

sdk/cosmosdb/cosmos/src/queryExecutionContext/Aggregators/MaxAggregator.ts

@@ -24,6 +24,11 @@ export class MaxAggregator implements Aggregator {
    * Add the provided item to aggregation result.
    */
   public aggregate(other: MaxAggregateResult): void {
+    // Skip aggregation if other.max is undefined (empty partition with count:0)
+    if (other.max === undefined) {
+      return;
+    }
+
     if (this.value === undefined) {
       this.value = other.max;
     } else if (

sdk/cosmosdb/cosmos/src/queryExecutionContext/Aggregators/MinAggregator.ts

@@ -24,6 +24,11 @@ export class MinAggregator implements Aggregator {
    * Add the provided item to aggregation result.
    */
   public aggregate(other: MinAggregateResult): void {
+    // Skip aggregation if other.min is undefined (empty partition with count:0)
+    if (other.min === undefined) {
+      return;
+    }
+
     if (this.value === undefined) {
       // || typeof this.value === "object"
       this.value = other.min;

sdk/cosmosdb/cosmos/test/public/integration/aggregateQuery.spec.ts

@@ -404,4 +404,32 @@ describe("Aggregate Query", { timeout: 20000 }, () => {
       }
     }
   });
+
+  it("should return both MIN and MAX values with empty partitions", async () => {
+    // Regression test: MIN aggregate was incorrectly overwritten to undefined
+    // when processing empty partitions (count:0) due to missing guard clause
+    const multiPartitionContainer = await getTestContainer(
+      "MIN MAX aggregate with empty partitions",
+      undefined,
+      {
+        id: "minmaxemptypartitions",
+        partitionKey: { paths: ["/pk"] },
+      },
+      { offerThroughput: 12000 }, // Force multiple physical partitions
+    );
+
+    // Create documents distributed across partitions (some partitions will be empty)
+    await multiPartitionContainer.items.create({ id: "1", pk: "partition1", date: "2025-01-15" });
+    await multiPartitionContainer.items.create({ id: "2", pk: "partition2", date: "2025-09-25" });
+
+    const queryIterator = multiPartitionContainer.items.query(
+      "SELECT MIN(c.date) AS minDate, MAX(c.date) AS maxDate FROM c",
+      { maxItemCount: 1 },
+    );
+    const { resources } = await queryIterator.fetchAll();
+
+    assert.equal(resources.length, 1, "Should return exactly one aggregate result");
+    assert.equal(resources[0].minDate, "2025-01-15", "MIN value should not be lost");
+    assert.equal(resources[0].maxDate, "2025-09-25", "MAX value should be present");
+  });
 });