Fix Pester 5 TagFilter, Health tests invoke actual functions, update Pattern C docs

[Copilot]

Fixes three open review comments from PR #606's review thread.

Description

Summary of changes:

• tests/offline/RunTests.ps1: Changed $pesterParams.Tag → $pesterParams.TagFilter. Pester 5 uses TagFilter; the old key silently added an unsupported parameter to Invoke-Pester, causing tag-filtered runs to fail before executing any tests.
• tests/offline/Health.Tests.ps1: Replaced fixture-only validation tests with tests that actually invoke Test-SdnResourceProvisioningState and Test-SdnResourceConfigurationState inside InModuleScope SdnDiag.Health with Mock Get-SdnResource. The old tests would remain green if the health functions were removed or broken. Get-SdnResource is mockable in Health's scope because SdnDiag.Health.psm1 explicitly imports SdnDiag.NetworkController.psm1. Tests now cover FAIL/PASS/WARNING branches and the guard clause that skips config-state checks when provisioningState is not Succeeded.
• tests/CONTRIBUTING_TESTS.md: Updated Pattern C from "validate fixture data structures" to the correct InModuleScope SdnDiag.Health + Mock Get-SdnResource invocation pattern with explanation.

# New Health.Tests.ps1 pattern
Describe 'Health - Test-SdnResourceProvisioningState' {
    It "Returns FAIL when resource has Failed provisioning state" {
        InModuleScope SdnDiag.Health {
            Mock Get-SdnResource {
                return $Global:PesterOfflineTests.SdnApiResourcesByRef['/servers/DVLAB-S1-N04']
            }
            $result = Test-SdnResourceProvisioningState -Resource 'Servers' -ResourceId 'DVLAB-S1-N04' -NcUri 'https://dvlab-nc.dvlab.contoso.local'
            $result.Result | Should -Be 'FAIL'
        }
    }
}

Change type

• Bug fix (non-breaking change)
• Code style update (formatting, local variables)
• New Feature (non-breaking change that adds new functionality without impacting existing)
• Breaking change (fix or feature that may cause functionality impact)
• Other

Checklist:

• My code follows the style and contribution guidelines of this project.
• I have tested and validated my code changes.