anthropic: Fix serialized representation of Effort::XHigh (#57985)

This PR fixes the serialized representation of the `Effort::XHigh`
variant, as it should be `xhigh`, not `x_high`.


https://platform.claude.com/docs/en/build-with-claude/adaptive-thinking#adaptive-thinking-with-the-effort-parameter

Release Notes:

- Fixed using `xhigh` thinking effort with Anthropic models.

crates/anthropic/src/anthropic.rs

@@ -676,6 +676,8 @@ pub enum Effort {
     Low,
     Medium,
     High,
+    #[serde(rename = "xhigh")]
+    #[strum(serialize = "xhigh")]
     XHigh,
     Max,
 }