Document Claude Opus 5 hosted model pricing (#62450)

The hosted-model reference now includes Claude Opus 5. This closes the
gap between the public documentation and the models that `cloud`
currently offers to Zed Pro and Zed Business customers.

The pricing table lists the provider price and Zed price for input,
output, cache-write, and cache-read tokens. The context-window table
lists the current 1M-token hosted limit. This change does not alter
model access or billing behavior.

Testing performed:

- `cd docs && npx prettier --check src/account/zed-hosted-models.md`
- `cd docs && mdbook build`

Release Notes:

- N/A