[codex] add checked process runner

[bobzhang]

Summary

Adds a high-level @process.run_checked wrapper for callers that want non-zero process exits to raise instead of manually checking the returned status code.

• adds @process.NonZeroExit with exit_code, cmd, and args
• keeps @process.run behavior unchanged: it still returns the child exit status
• documents the distinction between raw exit-code handling and checked execution
• adds native tests for success and non-zero checked execution

Validation

• moon check src/process --target native (passes; existing unrelated @buffer.new() deprecation warning in src/fs/tmpdir.mbt)
• moon test src/process/wait_test.mbt --target native (passes)
• moon test src/process --target native (passes on rerun; first run hit a transient orphan-cancellation timing failure, isolated rerun of that test passed)
• moon info (passes; reports existing/untracked warnings from examples/cancel_demo plus existing deprecation warnings)
• git diff --check

[coveralls]

Coverage Report for CI Build 354

Coverage decreased (-0.02%) to 79.333%

Details

• Coverage decreased (-0.02%) from the base build.
• Patch coverage: 3 of 3 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	4195
Covered Lines:	3328
Line Coverage:	79.33%
Coverage Strength:	80323.33 hits per line

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💛 - Coveralls