fix(api): guard stdio redirect when stdin is missing (closed fd 0)

[HoneyTyagii]

Summary

• Harden hindsight_api/daemon.py:_redirect_stdio_to_log() so a daemon child started with fd 0 closed no longer crashes.
• When fd 0 is closed at exec time, CPython sets sys.stdin to None, and the old unguarded sys.stdin.fileno() raised AttributeError: 'NoneType' object has no attribute 'fileno'.
• The embed spawner already pins stdin to DEVNULL after fix(embed): pin stdin to DEVNULL when spawning the daemon #4464, but the child redirect path still assumed a valid stdin, so direct launches and any other spawner without DEVNULL still hit the traceback from hindsight-embed: POSIX daemon spawn omits stdin=DEVNULL — child of a closed-fd0 parent dies in _redirect_stdio_to_log() (AttributeError: 'NoneType' object has no attribute 'fileno') #4580.

Root cause

Old code did:

sys.stdout.flush()
sys.stderr.flush()
with open(os.devnull, "r") as devnull:
    os.dup2(devnull.fileno(), sys.stdin.fileno())

If sys.stdin is None, the fileno() call crashes before the daemon can redirect stdio, leaving the "Failed to start daemon" 180s stall described in the issue.

Fix

• Skip the stdin redirect when sys.stdin is None or its fileno() raises.
• Wrap stdout and stderr flush and dup2 in the same None and OSError guards, so a closed or missing output stream cannot crash the redirect either.
• Behavior is unchanged when all streams are valid.

Verification

• python -m py_compile hindsight-api-slim/hindsight_api/daemon.py passes.
• ruff check hindsight-api-slim/hindsight_api/daemon.py passes.
• Mocked reproduction with sys.stdin=None: _redirect_stdio_to_log() completes without error and only redirects stdout and stderr (2 dup2 calls). Same for a stdin whose fileno() raises ValueError.
• Existing hindsight-api-slim/tests/test_daemonize.py covers parent and child paths; full pytest run was not possible here because the sandbox lacks a C toolchain for optional native deps, but the touched function has no signature change.

Fixes #4580

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[nicoloboschi]

Thanks for this! Closing as superseded: #4464 already fixed #4580 at the source — both spawners (hindsight_embed and hindsight_api/daemon.py) pin the child's stdin to /dev/null. The only way left to reach _redirect_stdio_to_log() with a closed fd 0 is to set the internal _HINDSIGHT_DAEMON_CHILD=1 env var by hand, which isn't a supported launch path.