ci: qualify Messages disconnect during built-in search

[franciscojavierarceo]

Summary

Add deterministic HTTP qualification for a client disconnect while a Messages built-in search is in flight. The test runs the actual gateway router over TCP with a temporary SQLite database and local inference/search servers. It verifies that the outbound search remains pending while the client is connected, is cancelled after disconnect, runs only once, starts no further inference round, and leaves no persisted Messages state.

Part of #110 and Enterprise Readiness #316. The developer guide documents the repeatable command and coverage limits. This covers local HTTP cancellation only; hosted ingress, PostgreSQL, live engines, OIDC, WebSocket cancellation, and rollback of remote tool side effects require separate evidence. No production behavior changes.

Test Plan

• cargo test -p agentic-server --test messages_stop_test --test messages_tool_disconnect_test: 13 passed.
• Re-ran the new disconnect test after adding the connected-client control assertion: passed.
• Workspace cargo clippy --all-targets -- -D warnings: passed.
• All-files pre-commit hooks, formatting, and diff checks: passed.
• Linux tests used the existing local static OpenSSL installation. No live inference service or external search service is contacted.