test(vulnerability_scanner): add acceptance tests and CI coverage

[maltere]

Add acceptance tests and Terraform fixtures for vulnerability scanner credential set, credential, and target environment resources, including update, no-drift, import, and sweeper coverage.

Wire these services into scripts/run-ci-tests so the new acceptance suites run in default CI acceptance runs.

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

Acceptance test run results

• I have added or updated acceptance tests for my changes
• I have run acceptance tests for my changes and included the results below

Steps to run acceptance tests

Test output

Additional context & links

[maltere]

Tested locally:

set -a; source .env; set +a; TF_ACC=1 go test \
  ./internal/services/vulnerability_scanner_credential/... \
  ./internal/services/vulnerability_scanner_credential_set/... \
  ./internal/services/vulnerability_scanner_target_environment/... \
  -run "^TestAcc" -v -count 1
=== RUN   TestAccVulnerabilityScannerCredential_Basic
--- PASS: TestAccVulnerabilityScannerCredential_Basic (19.00s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/internal/services/vulnerability_scanner_credential	21.301s
=== RUN   TestAccVulnerabilityScannerCredentialSet_Basic
--- PASS: TestAccVulnerabilityScannerCredentialSet_Basic (12.05s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/internal/services/vulnerability_scanner_credential_set	17.563s
=== RUN   TestAccVulnerabilityScannerTargetEnvironment_Basic
--- PASS: TestAccVulnerabilityScannerTargetEnvironment_Basic (12.56s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/internal/services/vulnerability_scanner_target_environment	16.491s