Add unit tests for Write-PSNowStructuredLog and Get-PSNowEnvironmentVariables helpers

[johnmccrae]

Problem

tests/Unit has coverage for the four main functions but no direct tests for Write-PSNowStructuredLog or the private environment helpers (GetPSNowOs, Get-PSNowTempDirectory). Bugs in these shared utilities would only surface indirectly through higher-level test failures.

Files

• Private/Write-PSNowStructuredLog.ps1
• Private/Get-PSNowEnvironmentVariables.ps1

Acceptance Criteria

• tests/Unit/Write-PSNowStructuredLog.tests.ps1 created with tests for: correct output format, multi-field entries, empty fields dict
• tests/Unit/Get-PSNowEnvironmentVariables.tests.ps1 created with tests for: Windows/macOS/Linux OS detection (mocked), temp path resolution, unsupported-OS error
• All new tests pass on both Windows and Linux CI runners
• Test count increases by at least 10