feat(gcp): add support for verifying Pub/Sub Topics and Subscriptions

[Amit2465]

Description

This PR adds support for Google Cloud Pub/Sub to the GCP module. It introduces helper functions to verify the existence of Pub/Sub Topics and Subscriptions, along with a complete Terraform example and automated integration tests.

This is a new feature addition and does not introduce any backward-incompatible changes.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.
• Make a plan for release of the functionality in this PR. If it delivers value to an end user, you are responsible for ensuring it is released promptly, and correctly. If you are not a maintainer, you are responsible for finding a maintainer to do this for you.

Release Notes (draft)

Added support for verifying Google Cloud Pub/Sub Topics and Subscriptions in the GCP module.

Migration Guide

No backward incompatible changes were introduced.

---

Verification Results

I have verified this PR locally against a real GCP project:

• Linting: Passed using golangci-lint run.
• Formatting: Passed go fmt and terraform fmt.
• Integration Test: Successfully ran TestTerraformGcpPubSubExample.

Test Output Snippet:

=== RUN   TestTerraformGcpPubSubExample
...
TestTerraformGcpPubSubExample 2026-03-08T16:21:20 logger.go:67: Verifying Pub/Sub topic pubsub-topic-mt4hzw exists in project amith-1772452782257
TestTerraformGcpPubSubExample 2026-03-08T16:21:21 logger.go:67: Verifying Pub/Sub subscription pubsub-sub-fzusep exists in project amith-1772452782257
...
--- PASS: TestTerraformGcpPubSubExample (49.30s)
PASS

[james00012]

A couple of minor, non-blocking suggestions: The strings.ToLower() calls in the test (lines 31-32) are unnecessary since random.UniqueId() already returns lowercase strings. You can drop those and the "strings" import.

Also, it'd be nice to see a negative test case (e.g., asserting that a non-existent topic returns an error), though that can be a follow-up. The Terraform version comment in main.tf saying "only being tested with 0.13.x" is stale, but that's consistent with other examples in the repo so not worth blocking on.

Overall this looks good to merge.

[james00012]

LGTM

[james00012]

You probably need to rebase the change anyway so if you can address those minor comments, it would be great.

[james00012]

You probably need to rebase the change anyway so if you can address those minor comments, it would be great.

Friendly ping in order for us to merge this change @Amit2465 .

[Amit2465]

Thanks for the feedback! Here's what was addressed:

• Removed the strings.ToLower() no-op calls and the unused strings import.
• Added negative test cases for both topic and subscription in modules/gcp/pubsub_test.go.
• Migrated to context-aware function signatures following the existing GCP module conventions.
• Rebased against main.

[james00012]

Thanks for addressing all the feedback, this looks good to merge. One minor note: TestAssertTopicAndSubscriptionExist largely duplicates TestAssertSubscriptionExistsNoFalseNegative; consider folding the subscription-topic linkage check into that test and dropping the duplicate. Not a blocker though. LGTM.

[james00012]

LGTM