fix: bound stdin closure during subprocess shutdown

[Oscar-Williams]

Summary

When a child stops reading stdin with buffered input pending, spawn_stdio_transport can hang before reaching its process-exit timeout. Close stdin and bound wait_closed() with shutdown_timeout, allowing the existing terminate/kill fallback to run while preserving graceful flushing.

Related issues

Applies the bounded stdin cleanup used by the Gemini example in #103 to the public transport helper.

Testing

• Linux / Python 3.14: same real-subprocess tests fail twice on base and pass all 5 cases with the fix; covers terminate, kill, full input delivery, body exceptions and an exited child.
• make test: 362 passed, 1 skipped (optional Gemini CLI).
• make check and make docs-test: passed.
• Windows / Python 3.11: 4 passed, 1 skipped in the new tests. The adjacent large-line test has an existing CRLF assertion failure on both base and head.

Docs & screenshots

Clarified timeout behavior in the function docstring. No UI or schema changes.

Checklist

• Conventional Commit title (e.g. feat:, fix:).
• Tests cover the change or are not required (explain above).
• Docs/examples updated when behaviour is user-facing.
• Schema regenerations (make gen-all) are called out if applicable.

The author set the contribution scope and validation requirements; Codex assisted with implementation and automated testing.