test: migrate JiraPS to Pester 6.2

[lipkau]

Summary

• Pin Pester 6.2.0 and allow test headers through the 6.x line.
• Update AtlassianPS.Standards to v0.4.0 and its immutable workflow action pins.
• Restore complete Examples and Help discovery in source and generated layouts.
• Keep the custom file-level integration runner and its failure accounting.
• Select deployment-specific Groups cases during Pester 6 discovery.

Compatibility and scope

This change preserves JiraPS runtime behavior, public interfaces, Cloud and Data Center behavior, Windows PowerShell 5.1 support, integration throttles, timeouts, workflow semantics, filtered mocks, and classic assertions.

The migration supersedes dependency PR #718, which remains open and untouched.

Validation

• Pester 5.9.1 baseline: 4,625 total, 4,556 passed, 0 failed, 69 skipped, 0 not run, 0 failed blocks, 0 failed containers.
• Pester 6.2.0: 4,693 total, 4,556 passed, 0 failed, 137 skipped, 0 not run, 0 failed blocks, 0 failed containers.
• Count change: 68 Examples cases are now explicitly discovered and skipped instead of producing an empty matrix.
• Source Help: 2,656 total, 2,588 passed, 68 skipped.
• Generated Release Help: 2,656 total, 2,588 passed, 68 skipped.
• Tests/Functions/Public/Invoke-JiraMethod.Unit.Tests.ps1: 67 total, 66 passed, 1 skipped.
• Custom runner pass/skip probe exited 0. Failure probes for test, block, and container failures exited 1.
• Merged NUnit probe: failed tests in failures=2; failed block plus failed container in errors=2; skipped case in skipped=1 and not-run=1.
• ./Tools/setup.ps1 passed outside the macOS sandbox after the documented PowerShellGet/.NET sandbox failure.
• Invoke-Build -Task Lint passed.
• Invoke-Build -Task Build, Test passed.
• actionlint .github/workflows/*.yml passed with actionlint 1.7.12.
• git diff --check passed.
• Edited PowerShell, Markdown, data, and XML files retain CRLF; workflow YAML retains LF.
• Independent review found no remaining issues.

CI and integration

• PR CI passed on Ubuntu, macOS, Windows PowerShell 7, and Windows PowerShell 5.1. The Cloud smoke job and aggregate CI Result also passed: https://github.com/AtlassianPS/JiraPS/actions/runs/34779873191
• Release Intent passed with release:none: https://github.com/AtlassianPS/JiraPS/actions/runs/34779872756
• Final Cloud and Data Center integration passed on commit 4619c07b: https://github.com/AtlassianPS/JiraPS/actions/runs/34780561207
• Cloud: 252 total, 241 passed, 0 failed, 11 skipped, 0 failed blocks, 0 failed containers.
• Data Center: 192 total, 178 passed, 0 failed, 14 skipped, 0 failed blocks, 0 failed containers.
• Cloud differs from the Pester 5 reference by +1 passed and -1 skipped. Pester 6 now runs both Cloud-specific Groups cases and skips the Server-specific case during discovery. Data Center matches the Pester 5 reference exactly and runs the Server-specific Groups case.
• An earlier final-commit Cloud attempt hit Atlassian's transient /rest/api/3/search removal response across independent suites. The immediate retry passed all Cloud tests without a code change.

Release intent

Build and test infrastructure only. No package release.

Refs AtlassianPS/AtlassianPS#12