anthropic: Deprecate fast mode for Opus 4.6 and Opus 4.7 (#61341)

TLDR: Opus 4.6 has been falling back to standard speed. Opus 4.7 fast
mode API will error after July 24th.

This will likely need to be cherry-picked to stable for either
Wednesday's release, or be pushed out separately on the 24th to maximize
availability. Although I don't think there are many Opus 4.7 fast mode
users, so Wednesday seems better to me.

> Fast mode for Claude Opus 4.7 is deprecated as of June 25, 2026, and
will be removed on July 24, 2026. After removal, requests to
claude-opus-4-7 with speed: "fast" will return an error; unlike Claude
Opus 4.6 (see the following note), Claude Opus 4.7 does not fall back to
standard speed. The model itself remains available at standard speed. To
continue using fast mode, migrate to Claude Opus 4.8.

Source: https://platform.claude.com/docs/en/build-with-claude/fast-mode

Release Notes:

- Deprecated Fast Mode for Opus 4.6 and Opus 4.7

crates/anthropic/src/anthropic.rs

@@ -22,10 +22,7 @@ pub const ANTHROPIC_API_URL: &str = "https://api.anthropic.com";
 pub const FAST_MODE_BETA_HEADER: &str = "fast-mode-2026-02-01";
 
 pub fn supports_fast_mode(model_id: &str) -> bool {
-    matches!(
-        model_id,
-        "claude-opus-4-6" | "claude-opus-4-7" | "claude-opus-4-8"
-    )
+    matches!(model_id, "claude-opus-4-8")
 }
 
 pub const FABLE_MODEL_ID_PREFIX: &str = "claude-fable-5";