chore(amplify): use the webhook.appId field instead of extracting it from the webhookArn

[carvantes]

Description

The Amplify GetWebhook API returns the appId field (docs). This changes the code to simply use the appId from the response instead of parsing the webhookArn to extract the appId.

There are no changes in functionality. This just simplifies the code to not make unnecessary assumptions on the ARN format. The motivation is to prevent reocurrence of of #39407 (comment)

Relations

Relates #39407

References

• https://docs.aws.amazon.com/amplify/latest/APIReference/API_GetWebhook.html

Output from Acceptance Testing

% make testacc TESTS=TestAccAmplify_serial PKG=amplify
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/amplify/... -v -count 1 -parallel 20 -run='TestAccAmplify_serial'  -timeout 360m -vet=off
2025/05/08 15:33:55 Initializing Terraform AWS Provider...
=== RUN   TestAccAmplify_serial
=== PAUSE TestAccAmplify_serial
=== CONT  TestAccAmplify_serial
=== RUN   TestAccAmplify_serial/Webhook
=== RUN   TestAccAmplify_serial/Webhook/basic
=== RUN   TestAccAmplify_serial/Webhook/disappears
=== RUN   TestAccAmplify_serial/Webhook/update
--- PASS: TestAccAmplify_serial (48.62s)
    --- PASS: TestAccAmplify_serial/Webhook (48.62s)
        --- PASS: TestAccAmplify_serial/Webhook/basic (15.00s)
        --- PASS: TestAccAmplify_serial/Webhook/disappears (12.98s)
        --- PASS: TestAccAmplify_serial/Webhook/update (20.63s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/amplify	53.928s

** Note: I commented out the tests not related to Amplify webhooks in this run. I believe that I cannot target only the Webhook tests with the TESTS parm due to how the TestAccAmplify_serial is set up https://github.com/hashicorp/terraform-provider-aws/blob/main/internal/service/amplify/amplify_test.go#L16

[github-actions]

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[github-actions]

✅ Thank you for correcting the previously detected issues! The maintainers appreciate your efforts to make the review process as smooth as possible.