Bound the reader-thread joins in Command

[ImLp]

Problem

Command.wait() joins its stdout and stderr reader threads with no timeout. A grandchild process that inherited the pipes keeps them open after the child exits, so the readers never see EOF and wait() blocks indefinitely, well past its own timeout argument.

Fix

• Both joins now honour reader_join_timeout, default 10s.
• readers_abandoned records that a join gave up, so a caller can tell.
• __exit__'s error path bounds the same two joins.

Scope

The reader threads are daemons and append to their output lists line by line, so giving up on them loses only trailing output from a process that is already being terminated. A clean exit still drains fully.

Testing

Two cases added to examples/test_command.py: a child that leaves a grandchild on its stdout made wait(timeout=5) return after 12.1s before this change and 2.5s after, and a clean exit still drains with readers_abandoned False. Full example suite: 32 passed, 3 skipped.

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