Keep stdin open for empty resumed SDK MCP input

[lakshminarayanannn]

Summary

• keep stdin open when an empty async prompt resumes a conversation that still needs bidirectional control responses
• preserve immediate close for new empty streams and prompt iterators that fail before writing
• cover the deferred SDK MCP handshake and both resume-aware client paths

Fixes #1226

Root cause

The zero-message branch added in 0.2.140 called transport.end_input() directly. That is correct for a new empty conversation, but a resumed deferred SDK MCP tool can issue a control request without a new user message. Closing stdin first made the SDK control response fail and the CLI returned tool_deferred_unavailable.

Test plan

• uv run python -m pytest tests/test_query.py::TestAsyncIterablePromptWithSdkMcpServers tests/test_query.py::TestCanUseToolKeepsStdinOpen::test_prompt_iterable_that_raises_immediately_closes_stdin -q — 7 passed
• uv run python -m pytest tests/ — 1488 passed, 6 skipped
• uv run python -m ruff check src/ tests/ scripts/ — passed
• uv run python -m ruff format src/ tests/ scripts/ --check — passed
• uv run python -m mypy src/ scripts/ — passed
• repeated the full unit, lint, format, and typecheck gates from a fresh clone at eca4a1ca508f090d8a53aa6201f9c734da9c641b
• ran the issue reproducer against current main: leg 2 returned tool_deferred_unavailable, is_error=True, duration_api_ms=0
• ran the same reproducer with this patch: leg 1 returned tool_deferred; leg 2 returned end_turn, is_error=False