fix(core): add | undefined to Transport optional callbacks for exactOptionalPropertyTypes compatibility

[rechedev9]

Summary

Fixes TS2420 errors when implementing the Transport interface in projects with exactOptionalPropertyTypes: true enabled.

The five optional callback/method properties on Transport previously lacked explicit | undefined:

// Before
onclose?: () => void;
onerror?: (error: Error) => void;
onmessage?: <T extends JSONRPCMessage>(message: T, extra?: MessageExtraInfo) => void;
setProtocolVersion?: (version: string) => void;
setSupportedProtocolVersions?: (versions: string[]) => void;

With exactOptionalPropertyTypes: true, TypeScript treats T | undefined and T as distinct in optional property contexts. A concrete class that declares onclose?: (() => void) | undefined is not assignable to an interface that declares onclose?: () => void under this flag, producing TS2420.

The fix adds explicit | undefined to all five optional members:

// After
onclose?: (() => void) | undefined;
onerror?: ((error: Error) => void) | undefined;
onmessage?: (<T extends JSONRPCMessage>(message: T, extra?: MessageExtraInfo) => void) | undefined;
setProtocolVersion?: ((version: string) => void) | undefined;
setSupportedProtocolVersions?: ((versions: string[]) => void) | undefined;

This is backwards-compatible: projects not using exactOptionalPropertyTypes are unaffected.

Breaking Changes

None.

Test plan

• Added packages/core/test/shared/transport.types.test.ts — compile-time type test verifying a class with explicit | undefined callbacks implements Transport without errors
• pnpm typecheck:all passes clean across all packages
• All 441 core tests pass

Fixes #1314

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commit: 08a2f28

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🦋 Changeset detected

Latest commit: 3052513

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/core	Patch

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