Fix notification debouncing for request ID zero

[Herrtian]

Summary

• treat numeric relatedRequestId: 0 as present when checking notification debounce eligibility
• preserve immediate send and rejection behavior for notifications associated with the first request ID
• add regression coverage and a patch changeset

Fixes #2117

Validation

• pnpm --filter @modelcontextprotocol/core-internal exec vitest run test/shared/protocol.test.ts
• pnpm --filter @modelcontextprotocol/core-internal exec vitest run --exclude test/wire/schemaTwinConformance.test.ts
• pnpm run typecheck:all
• pnpm run build:all
• pnpm exec changeset status --since origin/main

[changeset-bot]

🦋 Changeset detected

Latest commit: 4db24ae

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/server-legacy	Patch
@modelcontextprotocol/core	Patch
@modelcontextprotocol/codemod	Patch
@modelcontextprotocol/core-internal	Patch

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commit: 4db24ae