fix: normalize null to undefined in ElicitResultSchema content field

[mattzcarey]

Add z.preprocess to convert null values to undefined in the content field of ElicitResultSchema for better leniency while maintaining type compatibility.

Per MCP spec, content is typically omitted for decline/cancel actions but shouldn't error out for null values.

From the spec:

The three response actions are:

Accept (action: "accept"): User explicitly approved and submitted with data
- For form mode: The content field contains the submitted data matching the requested schema
- For URL mode: The content field is omitted
- Example: User clicked “Submit”, “OK”, “Confirm”, etc.

Decline (action: "decline"): User explicitly declined the request
- The content field is typically omitted
- Example: User clicked “Reject”, “Decline”, “No”, etc.

Cancel (action: "cancel"): User dismissed without making an explicit choice
- The content field is typically omitted
- Example: User closed the dialog, clicked outside, pressed Escape, browser failed to load, etc.

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1204

commit: d6adb78

[mattzcarey]

Fixes the issue which was originally noted here: #967

Closes #966