fix(tools): continue tool_runner loop on pause_turn with server_tool_use blocks

[s-zx]

Problem

When using client.beta.messages.tool_runner() with server-side tools (web_search, web_fetch) alongside @beta_tool client functions, the runner exits prematurely when the API responds with stop_reason="pause_turn" and server_tool_use blocks.

until_done() then returns a message whose last content block is a BetaServerToolUseBlock instead of text, causing AttributeError: 'BetaServerToolUseBlock' object has no attribute 'text'.

Closes #1170

Root Cause

_generate_tool_call_response() filters content for type == "tool_use" blocks. When the response contains only server_tool_use blocks, this filter returns nothing and the method returns None:

tool_use_blocks = [block for block in content if block.type == "tool_use"]
if not tool_use_blocks:
    return None   # ← exits here

The __run__ loop then sees None and exits:

response = self.generate_tool_call_response()
if response is None:
    log.debug("Tool call was not requested, exiting from tool runner loop.")
    return   # ← premature exit

Fix

Add _has_pause_turn_server_tools() to both BaseSyncToolRunner and BaseAsyncToolRunner. In __run__ (sync and async), check this helper before deciding to exit. When it returns True:

• Skip generate_tool_call_response() entirely
• Continue the loop without appending a new user turn — the server will resume on the next API call

if self._has_pause_turn_server_tools():
    log.debug("Received pause_turn with server_tool_use blocks; continuing...")
else:
    response = self.generate_tool_call_response()
    if response is None:
        return   # only exit when there are genuinely no tools