test: Make the psake/Invoke-Build task drift guard able to fail, in both directions

[tablackburn]

Summary

Fixes #215. The Invoke-Build Tasks drift guard has never once compared a task name, and now compares them in both directions.

Three defects had to be fixed, not the two the issue describes. Each on its own was enough to make the comparison vacuous, and they cancelled out into a passing test:

• $IBTasksResult was $null in the It that read it. It was assigned in a different It; the Describe-level $IBTasksResult = $null runs at discovery only, and Pester 5 does not carry a variable from one It into the next. The Invoke-Build job now runs in BeforeAll and both It blocks read $script:invokeBuildTaskName.
• Invoke-PSake -docs piped through Receive-Job returns format records, which have no Name property, so every psake task name was $null. Replaced with Get-PSakeScriptTasks, projected to strings inside the job before they cross the serialization boundary.
• Not in the issue: $IBTasksResult.All is an OrderedDictionary keyed by task name, so .All.Name was a lookup for a key called Name rather than an enumeration. Fixing only the two documented defects would still have compared psake's tasks against an empty list. The job now returns .All.Keys.

$null -notin $null is False, so the throw never fired, and an array of nulls satisfied Should -Not -BeNullOrEmpty.

The comparison is now bidirectional. A task defined in IB.tasks.ps1 and forgotten in psakeFile.ps1 leaves psake consumers without it — the same defect as #178 and #193 with the files swapped, and nothing else in the suite catches it. This matches the shape the signing comparison in this same file already uses. Each runner's own entry point is excluded on its own side: psake answers to default and ?, Invoke-Build to .. Nothing else is excluded.

Test plan

./build.ps1 -Task Test — green. Both directions were proved able to fail, not assumed:

Divergence introduced	Result
Task Sign renamed in IB.tasks.ps1	fails — but got 'Sign'
Task ExtraInvokeBuildTask appended to IB.tasks.ps1	fails — but got 'ExtraInvokeBuildTask'
neither	passes

Exactly one test fails in each case, so the neighbouring blocks are undisturbed.

The three other Describe blocks in this file — the settings comparison and README table checks from #178, and the signing settings comparison from #193 — are untouched.

Breaking changes

None. Test suite only.

No CHANGELOG.md entry: instructions/repository-specific.instructions.md scopes the changelog to user-facing changes to the shipped module and excludes the test suite. Nothing under PowerShellBuild/ changed.

Local variable names in the block being rewritten were expanded per shorthand.instructions.md ($outputModVerDir became $outputModuleVersionPath, and similar).

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Test Results

    4 files  ±0    889 suites  ±0   2m 6s ⏱️ -21s
  596 tests ±0    593 ✅ ±0   3 💤 ±0  0 ❌ ±0 
2 367 runs  ±0  2 295 ✅ ±0  72 💤 ±0  0 ❌ ±0 

Results for commit a28ac72. ± Comparison against base commit 3ea9e82.

This pull request removes 1 and adds 1 tests. Note that renamed tests count towards both.

Invoke-Build Tasks.Contains all the tasks that were in the Psake file

Invoke-Build Tasks.defines the same tasks in both task files

♻️ This comment has been updated with latest results.