Read each output stream to end of stream, not just until exit

[matt-edmondson]

Fixes #84

What changed

AsyncProcessStreamReader.Start used to loop while (!process.HasExited), finish the reads already in progress, and then make one more read of at most 4096 chars per stream. A process can exit with up to 64 KB per pipe still unread on Linux, so a large amount of output could be dropped. The exit code was still 0 and no error was raised.

Start now runs one read-to-EOF loop per stream (ReadToEnd), and the two loops run concurrently. Both are still raced against the caller's cancellation through the existing DrainOrAbandon/Abandon path, so a descendant that holds the pipe open still cannot hang a cancelled call. A failed read, such as a decode error, still reaches the caller, because DrainOrAbandon awaits both reads. That also removes the need for the old IsFaulted special case. ReadAndCallback/ReadCallback are folded into ReadToEnd, and the BOM stripping is unchanged.

CLAUDE.md's "Stream reading" section used to say that a final read after exit is enough to keep short-lived processes from losing output. It now describes the read-to-EOF behaviour.

Tests

ExecuteAsyncShouldDeliverOutputStillInThePipeWhenTheProcessExits writes 200,000 bytes through the existing GetEmitFileBytesCommand emitter (cat on Unix, PowerShell on Windows) and checks that every character arrives. It repeats this 10 times, because the loss depends on timing.

• With the fix temporarily reverted, the test fails on Linux/net10.0 ("Output was truncated on attempt 3").
• With the fix in place, the full suite passed on three consecutive runs: 42 passed, 2 skipped (the elevation tests that are Windows-only). The multi-target library build is clean.

This PR is independent of #86, which fixes #85 in LineOutputHandler. Each one branches from main.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CGpRxGxuvhstHo5zXQoWSA

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[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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