fix(core): treat relatedRequestId 0 as present in debounce guard

`0` is a valid JSON-RPC RequestId (and is also the first id a
Protocol instance uses). The debounce guard in `Protocol.notification`
used a truthy check `!options?.relatedRequestId` which treated 0 as
absent, and incorrectly coalesced related notifications that should
have bypassed debouncing.

Compare the asymmetry:

  | relatedRequestId | debounced? (pre-fix) |
  | 0                | YES (bug)            |
  | 1                | no                   |
  | "0"              | no                   |

Switch to `options?.relatedRequestId == null` - catches anull/undefined, preserves 0 as a valid id.

`relatedTask` is a structured object and cannot be falsy, so its guard
is unchanged.

Closes #2117.

Author: Aditya-Chilka

packages/core/src/shared/protocol.ts

@@ -1010,7 +1010,10 @@ export abstract class Protocol<ContextT extends BaseContext> {
         // A notification can only be debounced if it's in the list AND it's "simple"
         // (i.e., has no parameters and no related request ID or related task that could be lost).
         const canDebounce =
-            debouncedMethods.includes(notification.method) && !notification.params && !options?.relatedRequestId && !options?.relatedTask;
+            debouncedMethods.includes(notification.method) &&
+            !notification.params &&
+            options?.relatedRequestId == null &&
+            !options?.relatedTask;
 
         if (canDebounce) {
             // If a notification of this type is already scheduled, do nothing.

packages/core/test/shared/protocol.test.ts

@@ -768,6 +768,22 @@ describe('protocol tests', () => {
             expect(sendSpy).toHaveBeenCalledWith(expect.any(Object), { relatedRequestId: 'req-2' });
         });
 
+        it('should NOT debounce a notification that has relatedRequestId 0 (regression for #2117)', async () => {
+            // ARRANGE: id=0 is a valid JSON-RPC RequestId (and the first id a Protocol uses).
+            // The truthy guard \`!options?.relatedRequestId\` treated 0 as absent and
+            // incorrectly debounced these related notifications. Pin the fixed behaviour.
+            protocol = new TestProtocolImpl({ debouncedNotificationMethods: ['test/debounced_with_options'] });
+            await protocol.connect(transport);
+
+            // ACT
+            await protocol.notification({ method: 'test/debounced_with_options' }, { relatedRequestId: 0 });
+            await protocol.notification({ method: 'test/debounced_with_options' }, { relatedRequestId: 0 });
+
+            // ASSERT: both calls must reach the transport (no coalescing).
+            expect(sendSpy).toHaveBeenCalledTimes(2);
+            expect(sendSpy).toHaveBeenCalledWith(expect.any(Object), { relatedRequestId: 0 });
+        });
+
         it('should clear pending debounced notifications on connection close', async () => {
             // ARRANGE
             protocol = new TestProtocolImpl({ debouncedNotificationMethods: ['test/debounced'] });