fix(transport): forward max_turns=0 to the CLI instead of dropping it

[VihaanAgarwal]

What breaks

ClaudeAgentOptions(max_turns=0) is silently ignored. The built command omits --max-turns entirely, so the CLI runs with its default (no turn cap) instead of the 0 the caller asked for.

Confirmed against current main:

from claude_agent_sdk import ClaudeAgentOptions
from claude_agent_sdk._internal.transport.subprocess_cli import SubprocessCLITransport

t = SubprocessCLITransport(prompt="hi", options=ClaudeAgentOptions(max_turns=0))
t._cli_path = "/usr/bin/claude"
cmd = t._build_command()
print("--max-turns" in cmd)   # False  (should be True, value "0")

Root cause

_build_command guarded the flag with a truthiness check:

if self._options.max_turns:
    cmd.extend(["--max-turns", str(self._options.max_turns)])

max_turns is typed int | None = None, so None is the "unset" sentinel and 0 is a real caller value. Bare truthiness collapses the two. Every other numeric option in the same method already guards on is not None and forwards 0 correctly: max_budget_usd (0.0 is emitted), task_budget, max_thinking_tokens. max_turns was the lone exception.

A concrete way to hit it: a caller that computes a turn budget and decrements it (options.max_turns = max(0, remaining)) expects 0 to mean "stop", but instead gets an unbounded run.

Fix

Guard on is not None, matching the sibling numeric options, and let the CLI decide what 0 means rather than dropping it in the SDK.

Test

Added test_build_command_max_turns_zero in tests/test_transport.py: asserts --max-turns 0 is emitted for max_turns=0 and that None still emits no flag. It fails on the old truthiness check and passes with the fix. Full suite stays green (ruff, format, mypy clean).