Deprecate XCTest test helpers in favor of Swift Testing

[bkhouri]

Now that all test suites in the repository have been migrated from
XCTest to Swift Testing, mark every XCTest-flavored public helper in
ArgumentParserTestHelpers as @available(*, deprecated, message: ...)
so downstream users see a compile-time nudge toward the
Swift-Testing-native replacement.

Deprecated helpers and their replacements:

• TestableParsableArguments → TestableSwiftTestingParsableArguments
• TestableParsableCommand → TestableSwiftTestingParsableCommand
• XCTestExpectation.init(singleExpectation:) → TestExpectation
• AssertResultFailure → expectResultFailure
• AssertErrorMessage → expectErrorMessage
• AssertFullErrorMessage → expectFullErrorMessage
• AssertParse → expectParse
• AssertParseCommand → expectParseCommand
• AssertParseCommandErrorMessage → (no direct replacement; inline
  with #expect / Issue.record)
• AssertEqualStrings → expectEqualStrings
• AssertExecuteCommand → requireExecuteCommand
• AssertJSONEqualFromString → expectJSONEqualFromString
• assertSnapshot → expectSnapshot
• assertGenerateManual → expectGenerateManual
• assertGeneratedReference → expectGeneratedReference
• assertDumpHelp(type:) / assertDumpHelp(command:) →
  expectDumpHelp(type:) / expectDumpHelp(command:)
• XCTest.debugURL — no public replacement; the Swift Testing helpers
  derive it internally via a Bundle-marker class.

To keep the deprecated helpers callable in-file without emitting
cascading deprecation warnings, extract their shared implementation
into non-deprecated fileprivate helpers (_AssertExecuteCommand,
_assertEqualStrings, _assertSnapshot, _debugBundleURL). The
public deprecated overloads forward to these.

The six expect* helpers in TestHelpers+SwiftTesting.swift no
longer delegate to their Assert* counterparts — inlining their
Swift-Testing branches lets us deprecate the Assert* variants
without warning at those call sites.

requireHelp moves from TestHelpers.swift to
TestHelpers+SwiftTesting.swift (it was already Swift-Testing-native)
and now calls expectEqualStrings instead of AssertEqualStrings.

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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