[patch] Drop a reused handler's leftover partial line at the start of each run

[matt-edmondson]

Fixes #93

Problem

LineOutputHandler keeps unterminated text in outputBuffer/errorBuffer, and only Complete() clears it. RunAsync reaches Complete() only when a run succeeds. A run that was cancelled, or whose callback threw, left its partial line in the buffer, and reusing the handler glued that text onto the next run's first line. The issue's repro gives ["partialhello"] instead of ["hello"].

Change

• New internal virtual void Reset() on OutputHandler, a no-op by default. RunAsync calls it before it starts the process.
• LineOutputHandler.Reset() discards both buffers, including a trailing CR still waiting for a possible LF. A cancelled run's partial line isn't a real line, so it's dropped rather than delivered. The buffers are replaced rather than cleared, so a delivery still finishing from the old run can't touch the new run's buffer. That's why outputBuffer/errorBuffer are no longer readonly.

Abandoned readers, the issue's second point: I added a regression test where an orphaned descendant keeps the cancelled run's stdout open and writes to it while the next run is going. It passes with only the Reset change, because disposing the cancelled run's AsyncProcessStreamReader already ends that read, so the late write never reaches the handler. I tried a per-reader "abandoned" flag as well. No test failed without it, so I left it out to keep the change minimal.

Tests

Three new tests in RunCommandTests. They need sh and are inconclusive on Windows, like the existing shell-dependent tests:

• ReusedLineOutputHandlerShouldDropTheLineACancelledRunLeftUnfinished: the issue's repro. Cancel after printf partial arrives, then reuse the handler for echo hello, and expect hello.
• ReusedLineOutputHandlerShouldDropTheLineARunWhoseCallbackThrewLeftUnfinished: same, except the first run fails because the stderr callback throws.
• AbandonedReaderShouldNotDeliverIntoALaterRunOnTheSameHandler: an orphan writes late into the cancelled run's pipe while the next run is going, and the next run still sees only hello.

Checked locally (Linux, net10.0):

• With the Reset() call removed, all 3 new tests fail.
• The full suite passes with the fix (60 passed, 2 skipped elevation tests), and stayed green across 3 repeated runs.

🤖 Generated with Claude Code

https://claude.ai/code/session_01C9H64dhbitTL25J7BebFyE

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
9 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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