fix(core): prevent debouncing notifications when relatedRequestId is 0

[harshitj183]

Description

This PR fixes a bug in the protocol notification debounce guard where a relatedRequestId of 0 was incorrectly treated as falsy/absent.

In JSON-RPC/MCP, 0 is a valid request/notification identifier. It is also the default starting ID for Protocol instances in the TypeScript SDK.

Currently, the debounce guard logic uses a truthiness check:

const canDebounce = debouncedMethods.includes(notification.method) && !notification.params && !options?.relatedRequestId;

Because 0 is falsy, !options?.relatedRequestId evaluates to true when relatedRequestId is 0, causing the notification to be incorrectly debounced and coalesced.

Fix

We replace the truthiness check with an explicit undefined check:

const canDebounce =
    debouncedMethods.includes(notification.method) &&
    !notification.params &&
    options?.relatedRequestId === undefined;

This aligns the behavior of relatedRequestId: 0 with all other present IDs (such as strings or positive integers), ensuring they bypass the global debounce logic and are sent immediately.

Testing

1. Added a test case in packages/core-internal/test/shared/protocol.test.ts (should NOT debounce a notification that has a relatedRequestId: 0) that calls protocol.notification twice synchronously with relatedRequestId: 0 to verify that they are both sent immediately and not coalesced.
2. Ran pnpm test:all to verify that all workspace packages compile and all tests pass.

Resolves #2117

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