Modernize ConfluencePS test architecture

[lipkau]

Summary

• Split integration entrypoints under Tests/Integration with focused Spaces, Pages, Labels, Attachments, and Configuration files while preserving the existing ordered lifecycle through a shared integration helper.
• Move function tests into Tests/Functions/Public and Tests/Functions/Private with lightweight README scaffolding.
• Add a public cmdlet unit coverage baseline plus a Pester guard that fails on baseline omissions or covered cmdlets losing their test file.
• Update build test discovery so normal Test excludes Tests/Integration/**/*.Integration.Tests.ps1 and TestIntegration discovers the new integration layout while preserving Cloud/DataCenter/Smoke tag filtering.

Closes #222

Validation

• Invoke-Build -Task Lint
• Invoke-Pester -Path 'Tests/Functions'
• Invoke-Pester -Path 'Tests/Functions/Public/PublicCmdletUnitCoverage.Tests.ps1'
• Invoke-Pester -Path 'Tests/Integration/*.Integration.Tests.ps1' -Tag 'NoSuchTagForDiscoveryOnly'
• Invoke-Pester -Path 'Tests/Build.Tests.ps1'
• Invoke-Build -Task Build, Test

Integration

• Live Cloud/DataCenter integration tests were not run locally because integration environment variables/credentials are required.