refactor(sns): split publish topic fan-out into subscribers + per-protocol deliverers

[vieiralucas]

Summary

• Extract `collect_topic_subscribers` returning a `TopicSubscribers` struct with per-protocol `Vec`s (sqs, http, lambda, email, sms). The single `state.read()` now does one sweep instead of five adjacent inline filter chains.
• Extract `build_envelope_attrs` for the `Map<String, Value>` used in every SNS notification envelope.
• Introduce a `TopicFanoutContext` struct that carries the message / subject / envelope values each deliverer needs, so the helper signatures stay readable.
• Extract `deliver_to_sqs_subscribers`, `deliver_to_http_subscribers`, `deliver_to_lambda_subscribers`, `deliver_to_email_subscribers`, and `deliver_to_sms_subscribers` as methods on `SnsService`. The `publish` topic branch is now a short fan-out sequence that calls each helper in turn instead of inlining ~330 lines of delivery logic.

Test plan

• `cargo fmt`
• `cargo clippy --workspace --all-targets -- -D warnings`
• `cargo test --workspace --exclude fakecloud-e2e --exclude fakecloud-conformance`

---

Summary by cubic

Split SNS publish fan-out into a subscriber collection pass and per‑protocol deliverers. This simplifies publish, removes duplicate filtering, and gives a small performance win.

• Refactors
  • Added collect_topic_subscribers returning TopicSubscribers (one state.read() sweep).
  • Added build_envelope_attrs to build SNS envelope MessageAttributes.
  • Introduced TopicFanoutContext to pass shared data cleanly.
  • Extracted deliver_to_{sqs,http,lambda,email,sms}_subscribers on SnsService.
  • publish now calls these helpers; behavior is unchanged.

^{Written for commit 73639b1. Summary will update on new commits.}

[cubic-dev-ai]

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