Adds support for enabling Anthropic fast mode on configured models.

Previously, Anthropic models configured through
`language_models.anthropic.available_models` were always marked as not
supporting fast mode, so `speed: "fast"` would be stripped before
sending requests even when the configured model supported Anthropic fast
mode.

This adds an optional `supports_fast_mode` field to configured Anthropic
models. When enabled, the model is marked as supporting fast mode and
the required Anthropic beta header is added automatically. Built-in
fast-mode model detection remains the fallback when the setting is
omitted.

Testing:
- `cargo fmt --package anthropic --package language_models --package
settings_content`
- `cargo test -p language_models available_model --lib`
- `cargo test -p anthropic from_listed_enables_fast_mode --lib`
- `cargo check -p language_models`
- Manual: verified in a local build that a configured Anthropic model
can send fast mode requests correctly.

Release Notes:

- agent: Allow specifying if fast mode is supported for custom anthropic
models

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>