Honor the Run_ScriptBlock action input

[MariusStorhaug]

Context and request

Observed behavior: Run_ScriptBlock is transported from the workflow as a string and copied directly into Run.ScriptBlock. Pester 5.8.0 and 6.1.0 convert that string to an empty ScriptBlock[], so the supplied tests are discarded.

Expected behavior: A script block supplied through Run_ScriptBlock is executed, or an unsupported representation fails with a clear validation error.

Reproduction:

- uses: PSModule/Invoke-Pester@v3
  with:
    Run_ScriptBlock: |
      Describe 'inline tests' {
        It 'runs' { $true | Should -BeTrue }
      }

Environment: GitHub composite action with Pester 5.8.0 or 6.1.0.

Regression: Unknown. The declared action input appears not to have had a working scalar-to-script-block conversion.

Workaround: Put the test in a .Tests.ps1 file referenced by Path, or use a PowerShell configuration file containing a real script block.

Acceptance criteria

• Add an action-level regression test that fails because the current input is discarded.
• Convert the supported input representation to one or more ScriptBlock objects before creating the Pester configuration.
• Verify the inline tests execute with supported Pester versions.
• Document the accepted Run_ScriptBlock syntax and validation errors.

Technical decisions

The verified root cause is the scalar GitHub Action input crossing into a strongly typed Pester property without explicit conversion. Keep the correction limited to the action-input contract; configuration-file script blocks already retain their native type.

Implementation plan

1. Add the failing action-level regression test.
2. Define and validate the supported scalar representation.
3. Convert it explicitly before merging the Pester configuration.
4. Run the affected Pester 5 and Pester 6 action-test surfaces and update the input documentation.