Use predefined exception instead of arbitrary exit code in command modules

[zhenchaoni]

Summary

Fixes #509. Replaces bare sys.exit(N) / ctx.exit(N) / raise SystemExit(N) calls in command modules with predefined, typed exceptions so exit codes are managed consistently instead of scattered as magic numbers. No exit-code definitions change — codes 1/2/3/4 are preserved exactly.

Changes

• Added three typed exceptions in utils/cli.py:
  • ModelLoadError → exit 3 (model fails to load on device/EP)
  • InferenceError → exit 4 (prediction fails at runtime)
  • PartialSupportError → exit 1 (valid negative result; raised silently, no Error: prefix)
  • ModelLoadError/InferenceError override show() to print the message verbatim, preserving existing stderr output.
• analyze.py: removed sys import; all exits now raise UsageError (2) or PartialSupportError (1); added a re-raise guard so intentional exits aren't relabeled "Analysis failed".
• run.py: parse/format/file errors → UsageError (2); load → ModelLoadError (3); inference → InferenceError (4); success no-input path keeps ctx.exit(0).
• perf.py / serve.py: install/load failures → ClickException (1), with a re-raise guard in perf.

Exit code contract (unchanged)

Code	Meaning
0	success
1	negative result (partial support)
2	usage error
3	model load failure
4	inference failure

Testing

• Existing run tests asserting exit 3/4 are unchanged and still pass — behavior is fully guarded.
• 261 unit tests pass (run, run_spec, perf_cli, optracing, serve); no type/lint errors.