Harden cross-platform MSAL broker initialization

[jformacek]

MSAL broker initialization previously applied Windows-oriented settings across every platform and could silently select or miss incompatible native runtimes. This change makes broker behavior explicit per operating system and architecture while keeping non-broker authentication usable when a broker runtime is unsupported.

Changes

• Centralize Windows, Linux, and macOS broker configuration, including RID, native filename, redirect URI, account listing, and parent-window behavior.
• Load native broker runtimes lazily and require the exact platform asset, with actionable errors for missing files, load failures, and unsupported architectures.
• Configure only the current operating system in BrokerOptions; use the macOS unsigned-script redirect URI and avoid Windows parent-window handling there.
• Add a PowerShell 7 GitHub Actions matrix for Windows, Ubuntu, and macOS and make test paths platform-neutral.
• Document the supported broker OS and architecture combinations.

Validation

• Pester: 3 passed, 1 integration test skipped because credentials were not configured.
• Windows x64 broker factory construction and native runtime loading completed successfully.
• Module manifest validation completed successfully.

Full interactive broker authentication still requires an enrolled, interactive desktop and is outside the scope of hosted CI.