Feature: Implement js-libp2p Interop Tests for Echo Protocol

Description

Add a new interoperability test suite that validates py-libp2p against js-libp2p using the Echo protocol (/echo/1.0.0).

Motivation

As part of the Universal Connectivity initiative (2026 Roadmap), py-libp2p is expanding its interoperability coverage to ensure parity with other stack implementations.

Currently, the repository has:

• nim-libp2p: Echo protocol tests.
• go-libp2p & rust-libp2p: Active work on Ping/Echo (Issues #1136, #1142).

However, we are missing a dedicated Echo Protocol test for js-libp2p. Validating the Echo protocol is critical because it tests full bidirectional stream read/write capabilities (muxing, window management, payload integrity), whereas simple Ping tests often miss subtle stream handling bugs (e.g., Yamux hangs).

Requirements

1. JS Echo Server:

• Create a new script at tests/interop/js_libp2p/js_node/src/echo_server.js.
• This script should spin up a js-libp2p node listening on TCP that handles /echo/1.0.0 by piping the stream back to the source.

2. Python Test Harness:

• Create a new test file at tests/interop/js_libp2p/test_js_echo.py.
• Implement a pytest fixture to manage the JS daemon lifecycle (similar to the existing Nim implementation).
• Implement the test function using trio to dial the JS peer and verify payload integrity.

Open questions

None.

Are you planning to do it yourself in a pull request ?

Yes

Start by comparing the existing Nim interoperability implementation, then inspect the JS entry point at tests/interop/js_libp2p/js_node/src/echo_server.js and the Python harness at tests/interop/js_libp2p/test_js_echo.py. Add the JS daemon and pytest/trio lifecycle test so py-libp2p can dial /echo/1.0.0 and verify payload integrity through the echoed stream.