tests: mock-server.py is 755 bytes and appears to be a stub with no actual mock endpoints

[Haroldwonder]

Problem

mock-server.py is only 755 bytes. A real mock server for testing SEP-6 and SEP-10 flows would need to implement multiple endpoints (/deposit, /withdraw, /transaction, /auth). The current file is likely a placeholder that does not support any meaningful integration testing.

Location

mock-server.py

Fix

Implement a minimal Flask or http.server-based mock that handles the core SEP-6 endpoints (/deposit, /withdraw, /transaction) and the SEP-10 challenge endpoint (/auth), so integration tests can run against it.