Migrate remaining unit tests to Swift Testing

[bkhouri]

Migrate three ArgumentParserUnitTests suites from XCTest to Swift
Testing:

• HelpGenerationTests — the base file was already using @Suite,
  @Test, and #expect; drop the stale import XCTest. The
  +*.swift extension files were migrated in earlier PRs.
• ParsableArgumentsValidationTests — the largest unit-test file.
  One assertion required an update: comparing error.type (an
  existential any ParsableArguments.Type) against a concrete type
  fails to type-check under #expect's stricter operand resolution,
  so wrap both sides in ObjectIdentifier.
• CompletionScriptTests — marked @Suite(.serialized) because the
  tests share the CompletionShell.requesting global while
  generating completion scripts. Under XCTest the sequential runner
  hid this; Swift Testing's default parallelism caused cross-shell
  contamination in the generated scripts. Private
  assertCustomCompletion(s) ported to
  expectCustomCompletion(s).

Relates to #710

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

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