MCP connections gain an unused hook for adding per-request headers

What

MCP servers are outside tools and services Claude Code can connect to. For the MCP connections Claude Code sets up itself, a new way to add extra HTTP headers (small labelled values sent with each request) has been built in:

• The registration helper now accepts a requestHeaders option and stores it in a new cliOwnedRequestHeaderProviders WeakMap, next to the existing providers for the login token (resolveBearerToken) and for watching responses (onResponse).
• Both Design MCP request wrappers now call that provider with the request's method and body, and merge the headers it returns into each request. They never replace a header that is already set, and never replace Authorization.
• None of the code in this build that registers these configs passes requestHeaders; the callers pass only getBearerToken or nothing. So the hook exists but is not used yet.

Why

Nothing changes for users in this build. The hook makes it possible later to tag individual MCP requests with extra headers, and because the request body is handed to it, the headers can depend on what is being sent.