fix: support exactOptionalPropertyTypes in Transport interface

[naman10parikh]

Summary

Adds explicit | undefined to all optional callback properties on the Transport interface and all implementing classes/test mocks. This allows consumers with exactOptionalPropertyTypes: true in their tsconfig.json to use the SDK without type errors.

Problem

When a consumer enables exactOptionalPropertyTypes (a strict TypeScript config), class properties declared as onclose?: () => void are initialized to undefined by default. Under exactOptionalPropertyTypes, this means the property type becomes (() => void) | undefined, which is not assignable to () => void — causing TS2420 errors:

error TS2420: Class 'StreamableHTTPServerTransport' incorrectly implements interface 'Transport'.
  Types of property 'onclose' are incompatible.
    Type '(() => void) | undefined' is not assignable to type '() => void'.

Solution

Add | undefined to all optional properties in the Transport interface and all implementing classes:

// Before
onclose?: () => void;

// After
onclose?: (() => void) | undefined;

This is fully backwards-compatible — existing code that doesn't use exactOptionalPropertyTypes is unaffected.

Files Changed

• packages/core/src/shared/transport.ts — Transport interface (6 properties)
• packages/client/src/client/sse.ts — SSEClientTransport
• packages/client/src/client/streamableHttp.ts — StreamableHTTPClientTransport
• packages/client/src/client/websocket.ts — WebSocketClientTransport
• packages/client/src/client/stdio.ts — StdioClientTransport
• packages/core/src/util/inMemory.ts — InMemoryTransport
• packages/server/src/server/stdio.ts — StdioServerTransport
• packages/server/src/server/streamableHttp.ts — WebStandardStreamableHTTPServerTransport
• packages/core/test/shared/protocol.test.ts — MockTransport
• packages/core/test/shared/protocolTransportHandling.test.ts — MockTransport

Testing

• pnpm build:all passes with 0 errors
• No runtime behavior changes — this is a type-only fix

Fixes #1314

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