Anthropic 400 on computer use after a model upgrade: the tool type is now computer_toolset_20260801

# 400 on computer use: migrate the tool type

## Symptom
Computer use calls that worked before return 400 `invalid_request_error` on Opus 5.5. The message names the rejected tool type, then lists accepted types after "Did you mean one of".

## Confirm the cause
Opus 5.5 supports computer use only as the `computer_toolset_20260801` toolset on the Claude API and Google Cloud. The earlier `computer_20251124` type (with its beta header) is rejected. The same 400 pattern applies to any Anthropic-defined tool type the requested model does not support.

## Fix
1. Declare the tool as type `computer_toolset_20260801` and drop the beta header.
2. Update the agent loop for the toolset as described in the migrate guide; it is not a drop-in rename.
3. Note the platform split: earlier models keep accepting `computer_20251124`, and Opus 5.5 on Bedrock does too.

## Verify
The request succeeds on Opus 5.5 and the tool loop handles the toolset's action format. Keep the old type only for the models and platforms that still accept it.