Add connection manager tests

[quiloos39]

Summary

• Add 12 vitest tests for the global connection map in src/connections.ts
• Covers store/retrieve, delete, ready state tracking, multiple connections, same-ID replacement, iteration, cleanupAllConnections(), MAX_OUTPUT_BYTES constant, and StoredConnection interface shape

Test plan

• Run npx vitest run tests/connection-manager.test.ts and verify all 12 tests pass
• Confirm MAX_OUTPUT_BYTES matches the value in src/connections.ts