fix: treat relatedRequestId 0 as present in debounce guard

[juneja-varun]

Problem

The debounce guard in _notificationViaCodec used a truthiness check on options?.relatedRequestId:

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

0 is a valid JSON-RPC request id — and the first id a Protocol instance issues — but the truthy check treats it the same as "absent", so a notification related to request 0 gets silently coalesced by the debounce path even though related notifications are supposed to bypass debouncing.

Fix

Switch to an explicit presence check (!== undefined), matching how the rest of the codebase treats optional request ids.

Testing

Added a regression test mirroring the existing "should NOT debounce a notification that has a relatedRequestId" case, but with relatedRequestId: 0. pnpm --filter @modelcontextprotocol/core-internal exec vitest run test/shared/protocol.test.ts passes (40/40).

Fixes #2117

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🦋 Changeset detected

Latest commit: a81a911

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@modelcontextprotocol/client	Patch
@modelcontextprotocol/server	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/server-legacy	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core-internal	Patch

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commit: a81a911