Guard against drift between `plasterManifest.xml` and the `Invoke-Plaster` test stub

[raandree]

Problem description
PR #560 added unit tests in tests/Unit/Public/New-SampleModule.tests.ps1 that replace Invoke-Plaster with a hand-maintained stub listing every Plaster parameter statically. The stub is required because Invoke-Plaster declares manifest-driven parameters via dynamicparam, which Pester's auto-generated mock proxy cannot model.

When a new Use* toggle or parameter is added to Sampler/Templates/Sampler/plasterManifest.xml, the stub will silently drop it — $PSBoundParameters will not contain it and the tests will keep passing while real scaffolding behavior regresses.

Verbose logs
N/A

How to reproduce

1. Add a new <parameter name="UseSomething" ...> entry to Sampler/Templates/Sampler/plasterManifest.xml.
2. Do not update the stub in tests/Unit/Public/New-SampleModule.tests.ps1.
3. Run ./build.ps1 -Tasks test -PesterPath 'tests/Unit/Public/New-SampleModule.tests.ps1' -CodeCoverageThreshold 0.
4. Observe that tests pass even though the stub does not accept the new parameter.

Expected behavior
A test fails when the stub's param() block is out of sync with the parameters defined in plasterManifest.xml.

Current behavior
No such check exists; drift is silent.

Suggested solution
Add a lightweight guard test under tests/Unit/Public/ that:

1. Parses Sampler/Templates/Sampler/plasterManifest.xml.
2. Enumerates <parameter name="..."> nodes.
3. Asserts every name is present in (Get-Command Invoke-Plaster).Parameters.Keys within InModuleScope.

Consider also documenting the stub pattern in .github/instructions/test-writing.instructions.md.

Operating system the target node is running
N/A

PowerShell version and build the target node is running
N/A

Module version used
Sampler Unreleased / post-#560.