Add Network transport adapters

[rchatham]

Summary

Adds the next stacked Network migration slice: internal Network transport adapters that make the existing NetworkPeerCoordinator reachable through the transport factory seam, while keeping MultipeerConnectivity as the only public/default backend.

Changes

• Split MC-specific session requirements into MultipeerSessionTransport.
• Keep PeerSession conforming to the MC-specific transport protocol.
• Update MC-only consumers (PeerBrowser, advertiser assistant, invitation handler bridge) to require MultipeerSessionTransport.
• Add internal PeerConnectionTransportFactory.networkFramework.
• Add internal Network transport adapters:
  • NetworkPeerSessionTransport
  • NetworkPeerBrowserTransport
  • NetworkPeerAdvertiserTransport
  • NetworkPeerAdvertiserAssisstantTransport
• Add NetworkPeerListening / NetworkPeerBrowsing test seams.
• Allow Network connections/browsers to set handlers after construction so adapters can wire coordinator callbacks.
• Keep stream/resource transfer unsupported for Network with explicit internal errors instead of crashes.

Non-goals

• No public backend selector yet.
• No default backend change.
• No SwiftUI/browser UI changes.
• No local simulator loopback integration yet.
• Discovery still uses endpoint-derived temporary identities; PR Add Network transport adapters #27 will add Bonjour identity metadata.

Verification

• swift test — 84 tests passed
• xcodebuild test -project PeerConnectivity.xcodeproj -scheme PeerConnectivity -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.3.1' -configuration Debug — 84 tests passed

Stack context

Stacked on PR #25 (feature/network-migration-async-observers). This is intended to become PR #26 in the Network migration stack.

[Copilot]

Pull request overview

Adds internal Network.framework transport adapters and introduces a Multipeer-specific transport protocol split so MultipeerConnectivity-only components can depend on MC capabilities while keeping the default/public backend unchanged.

Changes:

• Split PeerSessionTransport into generic PeerSessionTransport + MC-specific MultipeerSessionTransport, and updated MC-only consumers accordingly.
• Added internal Network.framework adapter transports and a PeerConnectionTransportFactory.networkFramework factory for wiring them.
• Added Network transport adapter unit tests and updated the Xcode project to include the new sources/tests.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Sources/PeerSession.swift	Updates PeerSession to conform to the new MC-specific transport protocol.
Sources/PeerConnectionTransports.swift	Introduces MultipeerSessionTransport and adds the internal Network factory wiring.
Sources/PeerConnectionManager.swift	Gates multipeerSession access behind MultipeerSessionTransport and updates invitation handling signature.
Sources/PeerBrowser.swift	Requires MultipeerSessionTransport for MC browser construction.
Sources/PeerAdvertiserEventProducer.swift	Updates invitation handler to require MultipeerSessionTransport so it can access MCSession.
Sources/PeerAdvertiserAssisstant.swift	Requires MultipeerSessionTransport for MC advertiser assistant.
Sources/NetworkPeerTransportAdapters.swift	Adds Network.framework transport adapter implementations (session/browser/advertiser/assistant).
Sources/NetworkPeerTransport.swift	Adds Network browsing/listening seams and enables post-construction handler wiring.
PeerConnectivityTests/NetworkPeerTransportAdapterTests.swift	Adds coverage for Network adapter factory construction and basic adapter behaviors.
PeerConnectivity.xcodeproj/project.pbxproj	Registers new Network adapter source/test files in the Xcode project.

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[rchatham]

Superseded by consolidated PR #29. No commits were discarded; this branch is preserved as a recovery and reference point. Addressed review findings were replied to and resolved before consolidation.