fix(acp): expose Cursor non-fast model variants

[TheSentinel454]

Summary

• advertise ACP's parameterized model picker capability so Cursor exposes both fast modes
• present Cursor's fast=false and fast=true choices as model variants in the existing selectors
• validate and apply the base model before setting the Cursor fast parameter

Related issue

Fixes #6114.

Testing

• just ci
• cargo test -p buzz-acp --lib acp::tests::
• cargo test --manifest-path desktop/src-tauri/Cargo.toml commands::agent_models::tests::

Cursor CLI was not available locally, so this was verified with protocol fixtures rather than a live Cursor session.

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