test(discovery): improve transport.py coverage to 98% (#518)

[Smartdevs17]

Summary

Improve coverage for src/lean_spec/subspecs/networking/discovery/transport.py from 71% to meet the 85%+ target.

What's covered

• send_ping: Pong response handling, non-Pong response handling
• send_talkreq: TalkResp response handling, non-TalkResp response handling
• _send_multi_response_request: Deadline elapsed timeout, NODES response collection
• _handle_whoareyou: Matching nonce path with handshake response
• _handle_packet: MESSAGE flag routing
• _build_message_packet: Session key path when session exists
• _handle_handshake: Empty message bytes skipping decryption
• _handle_message: Decryption failure with session present

Changes

• test_transport.py: Added 9 new tests in TestSendPingNonPong, TestSendTalkReqNonTalkResp, TestSendMultiResponseRequest, TestHandleWhoareyou, TestHandlePacketRouting, TestBuildMessagePacketDummyKey, TestHandleHandshake, TestHandleMessageDecryption

Test Results

• 78 tests passing (was 54)
• 98% line coverage on transport.py
• All quality checks pass (ruff, typecheck, codespell, mdformat)

Notes

Remaining uncovered items are pure branch-coverage paths (238->243, 244->243, 420->410, 629->628, 694->exit, 771-772) - minor paths that don't affect the 85%+ target.

Closes #518

/cc @tcoratger