Add `from __future__ import annotations` to `empty_string_error.py`

[github-actions]

The module agents/agent1/src/python_agent_template/agents/agent1/validators/errors/empty_string_error.py is missing the from __future__ import annotations import at the top of the file.

The repo's coding standards explicitly require this future import in every module, and the other newly added validator error modules already follow this pattern. Omitting it can lead to annotation evaluation edge cases.

File: agents/agent1/src/python_agent_template/agents/agent1/validators/errors/empty_string_error.py
Line: 4

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This new module omits from __future__ import annotations, but the repo's coding standards explicitly call for it in every module (and the other newly added validator error modules follow that pattern). Add the future import to keep consistency and avoid annotation evaluation edge cases.

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Related PR: #51
Review comment: #51 (comment)

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