fix: use unbounded message buffer to prevent deadlock on multi-turn queries (#558)

[naga-k]

Problem

When using ClaudeSDKClient for multi-turn conversations, the second query() call hangs indefinitely after the first query involves subagent Task invocations.

Root Cause

The _read_messages() loop in Query handles both control protocol routing (init acknowledgments, tool result requests) and regular message buffering in a single async loop. The message buffer is bounded at 100 slots via anyio.create_memory_object_stream(max_buffer_size=100).

When receive_response() stops consuming at a ResultMessage, unconsumed messages (e.g., task_notification from subagent completion) remain in the buffer. If enough accumulate to fill the 100-slot buffer, _read_messages() blocks on await self._message_send.send(message).

This creates a deadlock chain:

1. _read_messages() blocked on buffer send → can't read stdout
2. CLI subprocess stdout pipe fills → CLI blocks on write
3. CLI can't process stdin → next query never processed
4. receive_response() hangs forever

Fix

Change max_buffer_size=100 to max_buffer_size=math.inf (unbounded). This is safe because:

• anyio.create_memory_object_stream explicitly supports math.inf for unbounded buffers
• Messages are still consumed by receive_response(), so memory stays bounded in practice
• The unbounded buffer just prevents the routing loop from stalling
• No behavioral change for normal single-turn usage

Changes

• 14 lines of production code (query.py): import math + unbounded buffer + explanatory comment
• ~200 lines of tests (test_message_buffer_deadlock.py): 3 tests that reproduce the deadlock with the old bounded buffer and verify the fix resolves it

Testing

• All 159 tests pass (156 existing + 3 new)
• test_bounded_buffer_blocks_control_message_routing — patches Query with buffer=100, sends 110 messages with no consumer, proves control request times out (deadlock reproduced)
• test_unbounded_buffer_allows_control_message_routing — same scenario with math.inf, control request succeeds (fix verified)
• test_query_class_uses_unbounded_buffer — asserts the config is math.inf
• Verified locally with real CLI: multi-turn conversations with tool use complete without hanging

Fixes #558