fix: improve torch install guidance for uvx/pipx

[fmueller]

Motivation

• Make the missing-PyTorch error more helpful by including installation hints for users who installed Scribae via uvx or pipx in addition to the existing uv/pip guidance.
• Ensure the test coverage asserts the new guidance so future regressions are caught.
• Avoid mypy import errors when torch is absent by loading it dynamically.

Description

• Updated the error raised in MTTranslator._require_torch in src/scribae/translate/mt.py to include uvx and pipx install instructions alongside the existing uv sync/pip hints.
• Switched to importlib.import_module("torch") and catch ModuleNotFoundError to keep type checking happy while still raising a helpful RuntimeError when PyTorch is not installed.
• Updated tests/unit/translate_pipeline_test.py to assert the presence of the new uvx and pipx hints in the missing-PyTorch error.

Testing

• Ran uv run ruff check and all lint checks passed.
• Ran uv run mypy and static type checks passed with no issues.
• Ran uv run pytest and the full test suite passed (160 passed).

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Codex Task