feat: add cloudflare_worker_secret migration support

[ssicard]

Summary

Adds v4-to-v5 migration for cloudflare_workers_secret and the deprecated cloudflare_worker_secret resources. In v5, standalone worker secrets no longer exist -- they are managed as secret_text bindings on the cloudflare_workers_script resource.

Migration behavior

• Handles both cloudflare_workers_secret (plural) and cloudflare_worker_secret (singular/deprecated
• Cross-resource merge: secrets are automatically folded into their parent workers_script bindings list when both are in the same file)
  Output of migrations hows a warning, since a deletion occurs:

⚠ Resource removed: cloudflare_workers_secret.literal_match_secret

The cloudflare_workers_secret resource has been removed in v5. Secrets are now managed as
bindings on the cloudflare_workers_script resource (script_name = "cftftest-literal-match").

If the parent cloudflare_workers_script is in the same file, the secret has
been automatically merged into its bindings list. Otherwise, add the
following binding to the parent resource manually:

  {
    type = "secret_text"
    name = "LITERAL_SECRET"
    text = "literal-secret-value"
  }

A 'removed' block has been generated to clean up the state entry during the next apply.

Note: In v4, secrets were managed via a separate API and could be updated
without redeploying the Worker script. In v5, secret_text bindings are part
of the script resource, so future secret changes will trigger a script
redeployment.

E2E Test Results

========================================
✓ E2E Test Complete!
========================================

Summary:

  Step 1: v4 terraform apply
    Status: ✓ SUCCESS

  Step 2: Migration (v4 → v5)
    Status: ✓ SUCCESS

  Step 3: v5 plan (before apply)
    Status: ✓ SUCCESS - Drift matched exemptions
    Result: 26 material changes (26 matched exemptions, 0 unmatched)
    Terraform: Plan: 0 to add, 5 to change, 0 to destroy.

  Step 4: v5 terraform apply
    Status: ✓ SUCCESS

  Step 5: v5 plan (after apply)
    Status: ✓ SUCCESS - Stable state achieved
    Result: No changes detected