Fix test reliability and DEBUG assertion correctness for &! operator

[Copilot]

Review feedback on the &! (ThreadJob background operator) implementation identified brittle test patterns and a broken DEBUG assertion.

Test fixes (ThreadJobBackgroundOperator.Tests.ps1)

• TypeName detection: Replaced $job.PSObject.TypeNames | Should -Contain 'ThreadJob' with $job.PSJobTypeName | Should -Be 'ThreadJob' — PSJobTypeName is the canonical property; TypeNames is implementation-specific and unreliable for this check
• Job cleanup: Wrapped all runtime It blocks in try/finally with Remove-Job -Force -ErrorAction Ignore so jobs are removed even on assertion failure
• Hang prevention: All Wait-Job calls now use -Timeout 30; tests throw explicitly if the timeout is hit rather than blocking the suite indefinitely

$job = Write-Output "ThreadJob Test" &!
try {
    $job.PSJobTypeName | Should -Be 'ThreadJob'
    $completedJob = $job | Wait-Job -Timeout 30
    if (-not $completedJob) {
        throw "Job did not complete within the allotted timeout (30 seconds)."
    }
}
finally {
    if ($null -ne $job) { $job | Remove-Job -Force -ErrorAction Ignore }
}

Token table assertion fix (token.cs)

The DEBUG-only Diagnostics.Assert calls validating s_staticTokenFlags and s_tokenText table lengths were still anchored to TokenKind.Clean (170) as the last enum value. With AmpersandExclaim = 171 appended, these assertions would fire in DEBUG builds. Updated to use TokenKind.AmpersandExclaim as the sentinel.

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