fix(sns): make Subscribe idempotent for same topic+protocol+endpoint

[naile]

Summary

Subscribe always created a new subscription, even when one already existed for the same TopicArn+Protocol+Endpoint. This caused duplicate subscriptions to accumulate across test re-runs, leading to duplicate message delivery.

Type of change

• Bug fix (fix:)
• New feature (feat:)
• Breaking change (feat!: or fix!:)
• Docs / chore

AWS Compatibility

Real AWS Subscribe is idempotent — calling it with the same TopicArn, Protocol, and Endpoint returns the existing subscription ARN. Verified against AWS documentation and LocalStack implementation.

There are two real AWS behaviors we don't cover:

• Attribute conflict rejection: If you re-subscribe with different attributes (FilterPolicy, RawMessageDelivery,
  etc.), AWS throws InvalidParameterException.
• Attribute pass-through: Subscribe accepts Attributes — floci's handlers don't parse them yet.

Both are irrelevant until we add attribute support to the Subscribe handlers.

Companion compatibility test: hectorvent/floci-compatibility-tests#38

Checklist

• ./mvnw test passes locally
• New or updated integration test added
• Commit messages follow Conventional Commits

[Copilot]

Pull request overview

This PR aligns Floci’s SNS Subscribe behavior with AWS by making it idempotent for the same TopicArn + Protocol + Endpoint, preventing duplicate subscriptions (and duplicate deliveries) across repeated runs.

Changes:

• Add an idempotency check in SnsService.subscribe to return an existing matching subscription instead of creating a new one.
• Add a unit test covering idempotent subscribe and distinct-endpoint behavior.
• Add an integration test covering idempotent subscribe via the SNS Query (form-encoded) protocol.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/main/java/io/github/hectorvent/floci/services/sns/SnsService.java	Adds lookup-before-create logic so Subscribe returns an existing subscription for the same topic/protocol/endpoint.
src/test/java/io/github/hectorvent/floci/services/sns/SnsServiceTest.java	Adds unit tests validating idempotency and distinct subscriptions for different endpoints.
src/test/java/io/github/hectorvent/floci/services/sns/SnsIntegrationTest.java	Adds query-protocol integration coverage for idempotent Subscribe and ensures no duplicate <member> entries are returned.

Comments suppressed due to low confidence (1)

src/test/java/io/github/hectorvent/floci/services/sns/SnsIntegrationTest.java:161

• @Order(7) is now used by both subscribe_idempotent and listSubscriptionsByTopic. JUnit does not guarantee a stable ordering between tests with the same order value, which can make the suite nondeterministic as it grows. Consider giving these tests distinct order values (or removing ordering reliance if possible).

    @Test
    @Order(7)
    void subscribe_idempotent() {
        String arn = given()
            .contentType("application/x-www-form-urlencoded")
            .formParam("Action", "Subscribe")
            .formParam("TopicArn", topicArn)
            .formParam("Protocol", "sqs")
            .formParam("Endpoint", sqsQueueUrl)
        .when()
            .post("/")
        .then()
            .statusCode(200)
            .extract().xmlPath()
                .getString("SubscribeResponse.SubscribeResult.SubscriptionArn");

        assert arn.equals(subscriptionArn) : "Expected existing subscription ARN but got a new one";

        given()
            .contentType("application/x-www-form-urlencoded")
            .formParam("Action", "ListSubscriptionsByTopic")
            .formParam("TopicArn", topicArn)
        .when()
            .post("/")
        .then()
            .statusCode(200)
            .body(containsString("<member>"))
            .body(not(containsString("</member><member>")));
    }

    @Test
    @Order(7)
    void listSubscriptionsByTopic() {
        given()

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/test/java/io/github/hectorvent/floci/services/sns/SnsIntegrationTest.java:151

• @Order(7) is now used by both subscribe_idempotent and listSubscriptionsByTopic. With OrderAnnotation, JUnit does not guarantee a deterministic execution order between tests that share the same order value, which can make the suite harder to reason about and potentially flaky as more stateful tests are added. Consider assigning a unique order and shifting subsequent order values accordingly.

    @Test
    @Order(7)
    void subscribe_idempotent() {
        String arn = given()
            .contentType("application/x-www-form-urlencoded")
            .formParam("Action", "Subscribe")
            .formParam("TopicArn", topicArn)
            .formParam("Protocol", "sqs")
            .formParam("Endpoint", sqsQueueUrl)
        .when()
            .post("/")
        .then()
            .statusCode(200)
            .extract().xmlPath()
                .getString("SubscribeResponse.SubscribeResult.SubscriptionArn");

        assert arn.equals(subscriptionArn) : "Expected existing subscription ARN but got a new one";

    }

    @Test
    @Order(7)
    void listSubscriptionsByTopic() {
        given()