fix(sns): support SubscriptionConfirmation and UnsubscribeConfirmation message types

[abhu85]

Summary

Fix SnsMessage and SnsMessageObj structs failing to deserialize SubscriptionConfirmation and UnsubscribeConfirmation SNS message types.

Root Cause:
The structs were designed only for Notification messages:

1. unsubscribe_url was a required String, but this field is only present in Notification messages
2. subscribe_url and token fields (present in confirmation messages) were missing entirely

Fix:

• Make unsubscribe_url optional (Option<String>) with #[serde(default)]
• Add subscribe_url field (Option<String>) for SubscriptionConfirmation/UnsubscribeConfirmation messages
• Add token field (Option<String>) for confirmation messages
• Apply same changes to both SnsMessage and SnsMessageObj<T> structs

SNS Message Type Field Presence

Field	Notification	SubscriptionConfirmation	UnsubscribeConfirmation
unsubscribe_url	Yes	No	No
subscribe_url	No	Yes	Yes
token	No	Yes	Yes

Test Plan

• Added test fixture example-sns-subscription-confirmation.json with real SubscriptionConfirmation payload
• Added my_example_sns_subscription_confirmation test verifying:
  • Successful deserialization of SubscriptionConfirmation messages
  • subscribe_url and token fields are correctly populated
  • unsubscribe_url is None for confirmation messages
  • Round-trip serialization/deserialization works
• Added my_example_sns_notification_has_unsubscribe_url test verifying:
  • Existing Notification messages still work correctly
  • unsubscribe_url is Some for Notification messages
  • subscribe_url and token are None for Notification messages
• All 212 existing tests pass

$ cargo test --features sns
running 212 tests
...
test result: ok. 212 passed; 0 failed; 0 ignored

Breaking Change Notice

This is a breaking change for code that directly accesses unsubscribe_url:

// Before (breaks for SubscriptionConfirmation)
let url: String = sns_message.unsubscribe_url;

// After (works for all message types)
let url: Option<String> = sns_message.unsubscribe_url;
if let Some(url) = sns_message.unsubscribe_url {
    // Use url
}

Fixes #966

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Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com