Add Test-Yaml function to validate YAML content

[MariusStorhaug]

Why

Automation that consumes YAML input often needs to know whether content is valid before acting on it, without wrapping ConvertFrom-Yaml in a try/catch just to check validity.

What

Add a Test-Yaml function that validates whether a given string or file content is well-formed YAML.

Acceptance criteria

• Test-Yaml returns $true for valid YAML input and $false for invalid input, without throwing
• Accepts input via pipeline or -InputObject, consistent with ConvertFrom-Yaml

[MariusStorhaug]

What changed: Reformatted the issue body to the MSXOrg Issue Format — a local Why plus an outcome-level What/acceptance criteria. Removed the Technical Decisions and Implementation Plan sections (and other implementation-tradeoff notes).
Why: Issue bodies should state local purpose and observable outcome without leaking implementation detail into the outcome-level description; durable technical decisions belong in the PR/design, not the tracking issue.
Follow-up: None.