Add integration tests for NAT peer subscription scenarios

[sanity]

Problem

The NAT address handling code in subscribe/seeding operations lacks end-to-end integration tests that verify the full flow works correctly with actual NAT scenarios.

Current Test Coverage

• ✅ Unit tests for LiveTransactionTracker::active_transaction_count
• ✅ Unit tests for connect operation NAT scenarios
• ✅ Unit tests for seeding manager subscriber operations
• ❌ No integration test for subscribe operation NAT address filling
• ❌ No end-to-end test showing NAT peer successfully subscribing through gateway

Suggested Tests

1. Subscribe NAT address filling test

   • Peer behind NAT sends subscribe request with PeerAddr::Unknown
   • Gateway fills in observed address from source_addr
   • Verify subscriber is registered with correct address

2. End-to-end NAT subscription test

   • Set up gateway + NAT peer topology
   • NAT peer subscribes to contract
   • Verify updates are correctly routed back to NAT peer

3. Edge case: source_addr is None

   • What happens if source_addr is unexpectedly None?
   • Should log warning and handle gracefully

Importance

Medium priority - The code paths are tested via unit tests and manual testing, but automated integration tests would catch regressions and document expected behavior.

Context

• Discovered during Claude bot review of PR fix: handle NAT address in subscribe and seeding operations #2193
• Could use freenet-test-network crate for integration testing

[AI-assisted - Claude]

[github-actions]

🤖 Auto-labeled

Applied labels:

• T-enhancement (95% confidence)
• E-medium (80% confidence)
• A-networking (90% confidence)
• A-developer-xp (88% confidence)

Reasoning: The report requests adding integration tests (not bug fixes or new runtime features), so it is an enhancement to test coverage. Implementing end-to-end NAT subscription tests using the freenet-test-network crate requires moderate effort to wire up gateway+NAT topologies and assertions (E-medium). The changes primarily involve networking behaviour (NAT, peer addressing) and improving developer testing/CI, so A-networking and A-developer-xp are appropriate.

Previous labels: none

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