test(argparse): use expect_error

[peter-jerry-ye]

Why

Argparse expected-failure tests used repetitive try/catch scaffolding even though the shared test helper provides @test.expect_error for this purpose.

What changed

Expected-error assertions across the argparse black-box suites now use @test.expect_error while retaining their exact error snapshots. The package adds the test-only helper import required by that API.

Why this is correct

Each converted test still asserts that the same operation raises and preserves the same expected error output. Expected-success paths are unchanged.

Scope

This is test-only cleanup; argparse behavior and public APIs are unchanged.

[coveralls]

Coverage Report for CI Build 6234

Coverage remained the same at 90.734%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	17969
Covered Lines:	16304
Line Coverage:	90.73%
Coverage Strength:	331814.11 hits per line

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💛 - Coveralls