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/assistant_context/src/assistant_context.rs

@@ -2117,6 +2117,7 @@ impl AssistantContext {
                                             );
                                         }
                                     }
+                                    LanguageModelCompletionEvent::RedactedThinking { .. } => {},
                                     LanguageModelCompletionEvent::Text(mut chunk) => {
                                         if let Some(start) = thought_process_stack.pop() {
                                             let end = buffer.anchor_before(message_old_end_offset);