fix: handle empty text_to_audio errors consistently

[cnYui]

Summary

• move the empty text_to_audio text validation inside the existing try block
• add a regression test that expects empty text to return a TextContent error instead of raising MinimaxRequestError

Fixes #88.

Verification

• .\.venv\Scripts\python.exe -m pytest -q — 7 passed
• .\.venv\Scripts\python.exe -m pytest --cov=minimax_mcp — 7 passed
• .\.venv\Scripts\ruff.exe check tests/test_server.py — passed
• git diff --check — passed

Notes

ruff check . still reports pre-existing lint issues in the repository, including the current HEAD version of server.py; this PR keeps the change scoped to the validation behavior and the regression test.