anthropic: Allow configured models to opt into fast mode

[Moeblack]

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

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[Moeblack]

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[dinocosta]

@Moeblack I might be missing something, but doesn't Zed already support fast model for these models? Using the latest stable release (1.4.4) I can see the toggle in the UI when selecting one of these models (Anthropic's Opus 4.6, 4.7, 4.8) , as well as confirm that it actually has a difference in speed, so I'm assuming it's working as intended.

[Moeblack]

@Moeblack I might be missing something, but doesn't Zed already support fast model for these models? Using the latest stable release (1.4.4) I can see the toggle in the UI when selecting one of these models (Anthropic's Opus 4.6, 4.7, 4.8) , as well as confirm that it actually has a difference in speed, so I'm assuming it's working as intended.我可能遗漏了什么，但 Zed 不是已经支持这些模型的快速模式了吗？使用最新的稳定版本（1.4.4），我可以在选择这些模型（Anthropic 的 Opus 4.6、4.7、4.8）时在用户界面看到切换选项，并且确认速度确实存在差异，所以我认为它正在按预期工作。

@dinocosta
Actually, the models you mentioned are currently hard-coded in the project. I'm working on some other anthropic models... so I submitted this fix.

[bennetbo]

Thank you!