fix(ai): keep Anthropic stream tool arguments after server tool blocks

[breken-ai]

💡 Motivation and Context

Anthropic stream events address content blocks by their index in the message. _AnthropicStreamAccumulator passed content_blocks to handle_anthropic_tool_delta and finalize_anthropic_tool_input, which look the block up with content_blocks[event.index]. But content_blocks only holds the block types the wrapper formats (text, thinking, redacted_thinking, tool_use). A server_tool_use or web_search_tool_result block still takes an index in the stream and is skipped in the list.

So once a response uses a server tool (web search, web fetch, code execution) before a client tool call, the list and the stream index no longer line up:

• the tool call's input_json_delta events point past the end of the list, and $ai_output_choices records the call with arguments: {};
• if a tool call came before the server tool, the server tool's input deltas land on the next tool block and its own arguments are replaced.

The fix keeps a second list that lines up with the stream index (an empty placeholder for block types that are not formatted) and uses it only for tool deltas and finalization. content_blocks and the captured output are unchanged.

💚 How did you test it?

• New test test_streaming_tool_call_after_server_tool_blocks builds the stream from the real anthropic.types events: server_tool_use (0), web_search_tool_result (1), text (2), tool_use with two input_json_delta chunks (3). It goes through the wrapped Anthropic client.
  • On main (4a138e6) it fails: the tool call is captured with 'arguments': {}.
  • With the fix it passes with {'city': 'SF'}.
• pytest posthog 4144 passed, 16 skipped. ruff format --check, ruff check, mypy through mypy-baseline filter and python -W error -c "import posthog" are clean.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

🤖 Agent context

Autonomy: Fully autonomous

• Found and fixed by an autonomous agent (Claude Code, Claude Opus) while reviewing the AI wrappers' streaming accumulators. The failing test was run against main before the fix.
• I kept the fix inside the accumulator instead of changing the handle_anthropic_* helper signatures, so the non-streaming path and the formatted output stay as they are.

Commits here are unsigned; this repo squash-merges, so GitHub signs the merged commit.

[dustinbyrne]

thanks @breken-ai

this looks correct to me (and if it is, we could benefit from the same strategy in posthog-js) - worth @PostHog/team-ai-observability signing off