fix: handle cancelled request id zero

[chenlichao]

Summary

• Treat requestId: 0 as a present cancellation target instead of returning early.
• Add a regression test covering notifications/cancelled for in-flight request id 0.

Fixes #2283.

Why this helps

0 is a valid JSON-RPC request id, and the SDK's own per-instance request counter starts at zero. The previous truthiness guard ignored cancellations for that first request, leaving its handler running.

Test Plan

• pnpm --filter @modelcontextprotocol/core-internal test -- test/shared/protocol.test.ts -t "request id 0"
• pnpm --filter @modelcontextprotocol/core-internal lint -- test/shared/protocol.test.ts src/shared/protocol.ts
• pnpm exec prettier --check packages/core-internal/src/shared/protocol.ts packages/core-internal/test/shared/protocol.test.ts
• git diff --check
• pre-push hook also ran pnpm -r typecheck, pnpm -r build, and pnpm -r lint

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Latest commit: 19c8c44

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