fix(server): reject requests before initialization

[he-yufeng]

Summary

• reject non-HTTP server requests before the MCP initialize -> initialized lifecycle completes
• keep initialize and ping available before initialization
• leave Streamable HTTP behavior unchanged because that transport already gates initialization itself

Refs modelcontextprotocol/servers#4195.

Validation

• pnpm --filter @modelcontextprotocol/server exec vitest run test/server/server.test.ts -t "rejects requests before the initialized notification"
• pnpm --filter @modelcontextprotocol/server test
• pnpm --filter @modelcontextprotocol/server typecheck
• pnpm --filter @modelcontextprotocol/server lint
• git diff --check
• pre-push hook: workspace typecheck, build, lint

[changeset-bot]

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Latest commit: 554a24b

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