Fix AMBA extended policy sync path handling

[Copilot]

Fixes #1309

AMBA extended policy sync was missing generated definitions on non-Windows runners because the file filters and path parsing assumed backslash-only paths. This update makes the extended-policy sync path handling cross-platform and adds coverage for syncing the secondary AMBA repository.

• Cross-platform extended policy discovery

  • Updated AMBA extended-policy file matching to recognize both /policy/ and \policy\ path separators.
  • Updated subpath extraction to parse nested service paths on Windows and Linux consistently.

• Regression coverage

  • Added a Pester test that runs Sync-ALZPolicyFromLibrary with -Type AMBA -SyncAMBAExtendedPolicies.
  • Verifies an extended policy from the AMBA repo is materialized into the expected EPAC policyDefinitions/AMBA/... location.

Example:

Sync-ALZPolicyFromLibrary `
  -DefinitionsRootFolder .\Definitions `
  -Type AMBA `
  -PacEnvironmentSelector "epac-dev" `
  -SyncAMBAExtendedPolicies