Isolate BundledAppWithSubDirs extraction from shared caches

[Copilot]

Persistent macOS Helix caches can cause an identical bundle to reuse an extraction on its first launch, violating the test’s new-extraction assertion.

• Isolation: Set DOTNET_BUNDLE_EXTRACT_BASE_DIR to a unique, fixture-owned directory for both variants. Retain the same path across launches to preserve reuse coverage and cleanup.
• Windows paths: Use extended-length local and UNC paths so the deeper extraction location supports the existing long-name content.
• Assertions: Verify that each extraction-enabled case creates the expected manifest directory beneath the isolated root.

• Fixes [ci-scan] Test failure: AppHost.Bundle.Tests.BundledAppWithSubDirs reuses a stale extraction on macOS Helix #135155

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