In the GitHub Actions CI environment, the runner has no SSH key configured
for github.e.it.census.gov, so 'terraform init' fails when pulling modules
whose source URLs use the SSH git@ scheme (e.g. tags.tf, examples/simple/eks.tf).

Changes made to .github/workflows/terraform-validate.yaml:

1. Added 'Setup GITHUB Credentials' step (CSVD/gh-auth@main)
   - Obtains a short-lived GitHub App token using the repository's
     GH_APP_PEM_FILE secret and GH_APP_ID / GH_APP_INSTALLATION_ID vars.
   - This mirrors the credential setup already present in the release workflow.

2. Added 'Configure Git HTTPS for Terraform module pulls' step
   - Runs: git config --global url."https://oauth2:<token>@github.e.it.census.gov/".insteadOf "git@github.e.it.census.gov:"
   - This tells git to transparently rewrite any SSH git@ URL for this host
     to an authenticated HTTPS URL at the transport layer.
   - Terraform shells out to the system git binary for module fetches, so
     it honours this rewrite without any changes to .tf source URLs.

No .tf files are modified: SSH source URLs in tags.tf and examples/simple/eks.tf
remain intact so that developers using SSH keys locally are unaffected.

Terraform normalizes SCP-style SSH sources (git@host:path) to ssh:// URLs
before invoking git, so the previous insteadOf rule for 'git@github.e.it.census.gov:'
did not match. Added a second rule for 'ssh://git@github.e.it.census.gov/' so
both URL forms are rewritten to authenticated HTTPS.

…form-validate action

- Action (CSVD/terraform-validate) now accepts github_app_pem_file and handles
  gh-auth + git config insteadOf internally
- Remove inline Setup GITHUB Credentials + Configure Git HTTPS steps from workflow
- Pass credentials directly to the action via 'with' inputs
- Resolves CSVDIES-10490