Fix truncated widget generation: raise model max_tokens to 64000

Root cause of the blank 3D canvas: langchain-anthropic's default
max_tokens (4096) truncated generateSandboxedUi tool args mid-stream —
css and html arrived but jsFunctions/jsExpressions were cut off, so the
middleware never emitted htmlComplete and the renderer never left its
preview sandbox (no final sandbox, no JS execution, no autosize). The
same truncation caused the earlier rebuild loop: each followUp turn saw
its own amputated tool call.

Diagnosed by replaying the exact streaming delta sequence against the
real renderer (passed — renderer was sound) and then extracting the live
activity content via React fiber: keys stopped at html with no
htmlComplete. Verified fixed live: full 19KB final frame, continuous
resize reports, working Three.js airplane with pitch/yaw/roll controls.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>