fix(pty): close master after child exit

[wan9chi]

Summary

• Transfer the PTY master to the child-monitor thread when the child exits.
• Close the master outside shared locks while the caller drains terminal output.
• Keep the EOF regression tests holding the writer alive and assert final output is preserved.

Motivation

Before Windows build 26100, ClosePseudoConsole can wait indefinitely unless its output pipe is drained concurrently. The current ownership cycle keeps the pseudoconsole handle in PtyWriter while PtyReader waits for EOF, causing direct PTY tests and the E2E harness to time out on Windows Server 2022 build 20348. Moving master teardown to the existing child-monitor thread follows Microsoft’s required close-and-drain ordering without truncating output.

[wan9chi]

• fix(pty): close master after child exit #519 👈 (View in Graphite)
• main

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[wan9chi]

Verified on Namespace Windows Server 2022 build 20348 in CI run 28994689990:

• All nine direct pty_terminal tests passed across the six default shards.
• cache_disabled::task_with_cache_disabled passed in 183 ms.
• The complete ignored Windows shard passed in 30 seconds.
• No 5-second PTY or 60-second E2E timeout remained.

The remaining immediate default-shard failures are the separately documented milestone screen-ordering issue. The temporary Namespace runner change has been removed from the PR.

[wan9chi]

Merge activity

• Jul 9, 5:03 AM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 9, 5:03 AM UTC: @wan9chi merged this pull request with Graphite.