[FEATURE] support `interleaved-thinking-2025-05-14` beta header through cli

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

I would like to run Claude Code with interleaved thinking for interactive long-running sessions.

Proposed Solution

When attempting to run claude with interleaved thinking beta headers we get a warning

claude --betas interleaved-thinking-2025-05-14 -p "what is 1+1?"
Warning: Beta header 'interleaved-thinking-2025-05-14' is not allowed. Only the following betas are supported: context-1m-2025-08-07

A simple support for the additional cli flag would be greatly appreciated