Update and redeploy a function

Replaces the function's source code with the body's code and triggers a redeploy. Same size limits as POST /functions. Use this verb to push secret writes into the running handler: passing the same code re-runs the deploy and refreshes the binding set with the latest values from the secrets table.

On deploy failure, the previously-deployed code stays live; the runtime never serves a half-built bundle. The response uses error.code deploy_failed, and the function's deploy_error field carries the latest deploy error for dashboard/API reads.