[jkendall327]

Summary

Centralizes repeated test doubles and helper infrastructure under Octans.Tests/Infrastructure.

• Moves the shared channel spy into the infrastructure namespace.
• Extracts repeated HTTP message handlers for simple stubs, downloader resolver tests, HTTP downloader unit tests, and download-manager integration tests.
• Extracts related test doubles for download content, disk-space checks, completion notification tracking, navigation, and throwing filesystem behavior.
• Updates affected tests to consume the centralized helpers while keeping purpose-specific HTTP fakes separate where they exercise different seams.

Validation

• dotnet build
• dotnet test