feat(custom_hostname_fallback_origin): add comprehensive lifecycle test

[ang-cloudflare]

SECENG-12969

• Replace basic single-step test with full lifecycle test covering Create, Drift check, Update, Drift check, and Import

• Upgrade CheckDestroy from v1 to v6 client

• Use statecheck pattern instead of TestCheckFunc

• Add ImportState testing

• Remove commented-out flaky update test (now covered by lifecycle test)

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

Changes being requested

• Add comprehensive lifecycle test for custom_hostname_fallback_origin resource
• Modernize test patterns to match current codebase standards

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

export TF_ACC=1
export CLOUDFLARE_API_KEY="<your-api-key>"
export CLOUDFLARE_EMAIL="<your-email>"
export CLOUDFLARE_ZONE_ID="<your-zone-id>"
export CLOUDFLARE_DOMAIN="<your-domain>"
go test -v -timeout=30m ./internal/services/custom_hostname_fallback_origin/... -run TestAccCloudflareCustomHostnameFallbackOrigin_FullLifecycle

Test output

=== RUN   TestAccCloudflareCustomHostnameFallbackOrigin_FullLifecycle
--- PASS: TestAccCloudflareCustomHostnameFallbackOrigin_FullLifecycle (12.77s)
PASS
ok      github.com/cloudflare/terraform-provider-cloudflare/internal/services/custom_hostname_fallback_origin    14.100s

Additional context & links