fix(tools): resume paused turns in the beta tool runner

[slegarraga]

Summary

client.beta.messages.tool_runner(...) exits the loop too early when a response contains only server-side tool use blocks (e.g. web_search, web_fetch) with stop_reason: "pause_turn". until_done() then returns the intermediate server_tool_use message instead of the model's final answer, so accessing e.g. response.content[0].text raises AttributeError.

Fixes #1170.

Root cause

After each turn, __run__ calls generate_tool_call_response(), which returns None whenever there are no client-side tool_use blocks to satisfy. The loop treated None as "the model is done" and returned — regardless of stop_reason. But "pause_turn" means the turn is still in progress (the server is running a long-running tool); per the documented contract you continue it by sending the partial response back:

"pause_turn": we paused a long-running turn. You may provide the response back as-is in a subsequent request to let the model continue.

— BetaMessage.stop_reason docstring

Fix

When there are no client-side tool calls and stop_reason == "pause_turn", the runner now appends the paused assistant message unchanged and continues the loop, only exiting on a genuinely terminal stop reason. Applied symmetrically to the sync (BaseSyncToolRunner) and async (BaseAsyncToolRunner) runners.

• The existing _messages_modified guard is preserved, so callers that manage their own message history are unaffected.
• pause_turn iterations increment _iteration_count, so any max_iterations ceiling bounds them exactly like regular tool-use iterations.

Tests

Added offline regression tests (sync + async, via respx) in tests/lib/tools/test_runners.py:

• test_pause_turn_resumes_loop_sync / test_pause_turn_resumes_loop_async — a pause_turn response (server tool only) followed by an end_turn answer; assert the runner makes the second request, returns the final text, and re-sends the paused assistant turn.
• test_pause_turn_chained_sync — two pause_turn responses before end_turn, covering a long server-tool wait.

These fail on main (the runner stops after the first response) and pass with the fix. ruff, pyright, and mypy are all clean.