fix: handle cancellation for request ID 0

[pshah19]

Summary

• Handle requestId: 0 as a valid JSON-RPC request ID in cancellation notifications
• Add regression coverage confirming that request handlers with ID 0 are aborted correctly

Root cause

The cancellation handler used a truthiness check for requestId, causing the valid numeric ID 0 to be treated as missing.

Testing

• pnpm --filter @modelcontextprotocol/core-internal exec vitest run test/shared/protocol.test.ts
• pnpm --filter @modelcontextprotocol/core-internal check
• pnpm lint:all
• pnpm test:all — 1 transient SSE timeout; the failed E2E scenario passed when rerun independently

Fixes #2283

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Latest commit: c2f4d2d

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