Add Context() method to network.Conn

[alienx5499]

Closes #3116

Summary

Implements Context() method for network.Conn that returns a context cancelled when the connection closes, enabling proper resource cleanup and early cancellation of dependent work.

Changes

• Added Context() method to network.Conn interface
• Implemented context management in swarm.Conn with lifecycle-aware cancellation
• Added Context() method to mock connections for testing
• Added comprehensive test suite covering all scenarios

Benefits

• Resource Cleanup: Clean up resources when connections close
• DoS Resiliency: Prevent resource leaks from closed connections
• Connection-Specific: Works per-connection, not per-peer (fixes the two-connection issue mentioned in [Proposal] .Context() for network.Conn (or maybe just swarm.Conn) #3116)
• Early Cancellation: Cancel work that depends on the connection

Testing

✅ All tests pass
✅ No regressions
✅ Build succeeds

Prior Art

Follows patterns from Go stdlib (*http.Request.Context(), *tls.ClientHelloInfo.Context()) and quic-go (quic.Connection.Context())