Fix assignee reference

[f3rnandomoreno]

This pull request includes several changes to improve the handling of task assignments and update the related tests and methods accordingly. The most important changes include updating method parameters and prompt text to use "assignee" instead of "userName," removing the lastUpdateByUser field, and adding new test cases to validate task assignment functionality.

Updates to method parameters and prompt text:

• src/main/java/com/fmoreno/telegramtaskaiagent/agents/ManagerAgent.java: Changed method parameters and prompt text to use "assignee" instead of "userName" in buildPrompt and other related methods. [1] [2] [3]
• src/main/java/com/fmoreno/telegramtaskaiagent/agents/NL2SQLAgent.java: Updated processNaturalLanguageToSQL method to use "assignee" instead of "userName."

Database model changes:

• src/main/java/com/fmoreno/telegramtaskaiagent/persistence/model/TaskEntity.java: Removed the lastUpdateByUser field from the TaskEntity class.

Test case additions and updates:

• src/test/java/com/fmoreno/telegramtaskaiagent/agents/NL2SQLAgentTestIT.java: Added new test cases to validate task creation with specific assignees.
• src/test/java/com/fmoreno/telegramtaskaiagent/persistence/TaskRepositoryTestIT.java: Added new test cases to validate task assignment updates and ensure correct assignee handling.

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