Expand Network backend E2E coverage

[rchatham]

Summary

Expands Network backend end-to-end coverage beyond the initial one-way loopback smoke test.

Changes

• Refactor NetworkPeerLoopbackTests to use shared helpers for service type, PSK security, and Network-backed manager creation.
• Keep the original secured discovery/connect/typed-message loopback coverage.
• Add secured bidirectional messaging coverage:
  • Alice sends a large typed payload to Bob.
  • Bob sends a typed reply to Alice.
• Add wrong-service isolation coverage to prove Network-backed managers on different Bonjour service types do not discover or connect to each other.
• Retain mismatched PSK negative coverage.

Verification

• swift test --filter NetworkPeerLoopbackTests — 4 tests passed
• swift test — 106 tests passed
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 106 tests passed
• GitHub Actions pull_request CI — Swift Package Tests and Xcode Project Tests passed
• GitHub Actions push CI — Swift Package Tests and Xcode Project Tests passed
• Correctness reviewer — no merge-blocking findings
• Security reviewer — no merge-blocking security findings

Notes / limitations

• These are real local Network.framework/Bonjour loopback tests, so they intentionally use bounded timeouts and unique per-run service types to reduce cross-run collisions.
• Multi-peer and disconnect/reconnect stress tests remain follow-up slices to avoid making this PR too flaky or broad.

Stack context

Stacked on PR #34 (feature/network-migration-api-expectations). This is the expanded E2E coverage slice of the Network migration stack.

[rchatham]

Superseded by consolidated PR #42. No commits were discarded; this branch is preserved as a recovery and reference point. Addressed review findings were replied to and resolved before consolidation.