refactor: remove archived tweet field from historical alerts

[esloch]

Description

Remove the archived tweet field from the historical alert records handled by AlertTools.

This change is part of the cleanup required before removing the physical tweet column from the historical alert tables in AlertaDengue.

Changes

• remove tweet from tabela_historico() output;
• stop writing tweet in write_alerta();
• remove the legacy casoscli$tweet <- NA placeholder;
• add focused tests covering dengue, chikungunya and zika historical alerts.

Validation

• focused test file passes: 15 passed;
• git diff --check passes;
• R/alert_functions.R has no active tweet references;
• generated historical alert SQL no longer references tweet;
• no database connection or SQL write was performed.

Notes

The full old testthat::test_dir("tests/testthat") suite still fails in pre-existing tests because some package functions are not loaded in the test environment: setCriteria, getCases, epiYear, and data2SE.

The new focused test file passes independently.

Out of scope

• no database migration;
• no production action;
• no AlertaDengue change;
• no AlertaDengueAnalise change;
• no unrelated AlertTools refactor.