fix: Add identity placeholders for existing principalId and clientId (#3972) - #3974

PR Summary

Fixes #3972.

Expanding a template fails when a nested module's name or a role assignment's name hashes the principalId (or clientId) of an existing user-assigned identity, for example uniqueString(grant.name, grant.principalId). The error is: "The argument 'additional parameters as needed' for 'UniqueString' is not a valid string", and the whole file is skipped.

Root cause: reference() on an existing identity that isn't part of the deployment returns an unknown mock for principalId/clientId. Once that mock is placed inside an object or array literal, such as a module parameter, it expands to null, so the nested uniqueString()/guid() gets a non-string.

Fix: add principalId and clientId of Microsoft.ManagedIdentity/userAssignedIdentities to the typed resource placeholder table introduced in #3908, so they resolve to a string. The placeholder is ffffffff-ffff-ffff-ffff-ffffffffffff, the value already used as PLACEHOLDER_GUID for identities deployed in the template, so existing and deployed identities now expand the same way. The expression functions stay strict, as in #3908.

Scope and trade-offs:

• Every existing identity gets the same placeholder, as deployed identities already do. Two of them hashed against the same scope produce the same name, and PSRule merges the duplicate resource IDs rather than failing.
• tenantId isn't added: its right value is the configured tenant, which the static table can't reach.
• A system-assigned resource.identity.principalId goes through a different path and isn't covered here.

Tests:

• New Tests.Bicep.46 fixture: an existing cross-resource-group identity, a module named ra-${uniqueString(grant.name, grant.principalId)}, and a role assignment named with guid(..., principalId, ...). Compiled with Bicep 0.48.1.
• New TemplateVisitorTests.ProcessTemplate_WhenExistingResourcePropertyPassedToModuleAndHashed_ShouldExpand and FunctionTests.ReferenceWithIdentityPlaceholders. Both fail without the fix and pass with it.
• BicepSymbolicNameTests: one pinned role assignment name changes because guid(principalId) now hashes the placeholder instead of an empty string.
• Full .NET test suite: 410 passed locally.

PR Checklist

• PR has a meaningful title
• Summarized changes
• Change is not breaking
• This PR is ready to merge and is not Work in Progress
• Other code changes
  • Unit tests created/ updated
  • Link to a filed issue
  • Change log has been updated with change under unreleased section