fix: keep related request id zero from debouncing

[he-yufeng]

Summary

• treat numeric relatedRequestId: 0 as present when deciding whether a notification can be debounced
• add regression coverage so related notifications for request id zero are sent immediately instead of being coalesced
• add a patch changeset for @modelcontextprotocol/core

Fixes #2117

To verify

• corepack pnpm --filter @modelcontextprotocol/core test -- --run packages/core/test/shared/protocol.test.ts
• corepack pnpm --filter @modelcontextprotocol/core typecheck
• corepack pnpm --filter @modelcontextprotocol/core lint
• git diff --check origin/main..HEAD

The pre-push hook also passed the repo typecheck/build/lint tasks.

[changeset-bot]

🦋 Changeset detected

Latest commit: ffe6967

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

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/core	Patch

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commit: ffe6967