Skip validation of FUNCTIONS_WORKER_RUNTIME with function metadata in placeholder mode

[kshyju]

Skip validation of FUNCTIONS_WORKER_RUNTIME with function metadata in placeholder mode (this validation was added in #10439)

Without this change, DNI apps in placeholder mode will log the error about runtime mismatch because the "WarmUp" function's Language is "CSharp".

Pull request checklist

IMPORTANT: Currently, changes must be backported to the in-proc branch to be included in Core Tools and non-Flex deployments.

• Backporting to the in-proc branch is not required
  • Otherwise: Link to backporting PR - will follow
• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• I have added all required tests (Unit tests, E2E tests)

Additional information

Additional PR information

[kshyju]

After consulting with the team, we decided that this change does not need to be backported to in-proc.