test: add MockSv2Client for connection manager testing

[xyephy]

Add a mock SV2 client that enables testing Sv2Connman without real TCP sockets.

This would allow testing:

• Rapid connect/disconnect scenarios
• Max client limit behavior
• Message injection for edge case testing
• Client cleanup races

[Sjors]

We have something like this in the test code?

[xyephy]

We have something like this in the test code?

Yes, ConnTester in src/test/sv2_connman_tests.cpp already does this using DynSock.

[Sjors]

Ok, so that could be moved to a more reusable place in the test code.