Synchronous call channel integration tests

[artugro]

Summary

Add integration tests for the synchronous call channel (POST /networks/{id}/call).

This was intentionally skipped in the initial test suite because calls require a real callback URL that responds synchronously. We need either:

1. A test agent in wisdom-agents that accepts call payloads and returns responses
2. A mock HTTP server spun up during tests

Depends on

• wisdom-agents: needs a test agent with a callback endpoint that responds to calls
• Backend: call channel implementation (already done in src/network/services/channels.py)

Test scenarios

• Call between two participants with real callback URLs
• Call timeout handling
• Call with context injection (verify context is included in payload)
• Call response recorded in network context