fix: add | undefined to transport option callbacks for exactOptionalPropertyTypes

[Christian-Sidak]

Summary

Adds | undefined to optional callback type properties in WebStandardStreamableHTTPServerTransportOptions to support TypeScript projects using exactOptionalPropertyTypes: true.

Fixes #1397

Changes

WebStandardStreamableHTTPServerTransportOptions (packages/server/src/server/streamableHttp.ts):

• sessionIdGenerator?: (() => string) | undefined
• onsessioninitialized?: ((sessionId: string) => void | Promise<void>) | undefined
• onsessionclosed?: ((sessionId: string) => void | Promise<void>) | undefined

Corresponding private fields on the class and the test helper interface are updated to match.

Why

Without this fix, users with exactOptionalPropertyTypes: true get:

error TS2379: Argument of type '{ sessionIdGenerator: undefined; enableJsonResponse: true; }'
is not assignable to parameter of type 'WebStandardStreamableHTTPServerTransportOptions' with
'exactOptionalPropertyTypes: true'.
  Types of property 'sessionIdGenerator' are incompatible.
    Type 'undefined' is not assignable to type '() => string'.

This is the standard TypeScript pattern for exactOptionalPropertyTypes compatibility. The Transport interface in @modelcontextprotocol/core already follows this pattern -- this PR brings the server transport options in line.

Testing

• tsc --noEmit passes for both packages/core and packages/server
• No behavioral changes -- purely a type-level fix

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