agent: Fix issue with Anthropic thinking models (#33317)

cc @osyvokon 

We were seeing a bunch of errors in our backend when people were using
Claude models with thinking enabled.

In the logs we would see
> an error occurred while interacting with the Anthropic API:
invalid_request_error: messages.x.content.0.type: Expected `thinking` or
`redacted_thinking`, but found `text`. When `thinking` is enabled, a
final `assistant` message must start with a thinking block (preceeding
the lastmost set of `tool_use` and `tool_result` blocks). We recommend
you include thinking blocks from previous turns. To avoid this
requirement, disable `thinking`. Please consult our documentation at
https://docs.anthropic.com/en/docs/build-with-claude/extended-thinking

However, this issue did not occur frequently and was not easily
reproducible. Turns out it was triggered by us not correctly handling
[Redacted Thinking
Blocks](https://docs.anthropic.com/en/docs/build-with-claude/extended-thinking#thinking-redaction).

I could constantly reproduce this issue by including this magic string:
`ANTHROPIC_MAGIC_STRING_TRIGGER_REDACTED_THINKING_46C9A13E193C177646C7398A98432ECCCE4C1253D5E2D82641AC0E52CC2876CB
` in the request, which forces `claude-3-7-sonnet` to emit redacted
thinking blocks (confusingly the magic string does not seem to be
working for `claude-sonnet-4`). As soon as we hit a tool call Anthropic
would return an error.

Thanks to @osyvokon for pointing me in the right direction 😄!


Release Notes:

- agent: Fixed an issue where Anthropic models would sometimes return an
error when thinking was enabled

crates/language_models/src/provider/anthropic.rs

@@ -554,9 +554,7 @@ pub fn into_anthropic(
                         }
                         MessageContent::RedactedThinking(data) => {
                             if !data.is_empty() {
-                                Some(anthropic::RequestContent::RedactedThinking {
-                                    data: String::from_utf8(data).ok()?,
-                                })
+                                Some(anthropic::RequestContent::RedactedThinking { data })
                             } else {
                                 None
                             }
@@ -730,10 +728,8 @@ impl AnthropicEventMapper {
                         signature: None,
                     })]
                 }
-                ResponseContent::RedactedThinking { .. } => {
-                    // Redacted thinking is encrypted and not accessible to the user, see:
-                    // https://docs.anthropic.com/en/docs/build-with-claude/extended-thinking#suggestions-for-handling-redacted-thinking-in-production
-                    Vec::new()
+                ResponseContent::RedactedThinking { data } => {
+                    vec![Ok(LanguageModelCompletionEvent::RedactedThinking { data })]
                 }
                 ResponseContent::ToolUse { id, name, .. } => {
                     self.tool_uses_by_index.insert(