Add python-dotenv to scaffolded conftest.py for integration tests

[JoeCardoso13]

Problem

Generated repos require users to export API keys in their shell to run integration tests. A .env file in the repo root (already in .gitignore and .mcpbignore) is silently ignored because conftest.py only checks os.environ.get().

Proposed fix

1. Add python-dotenv to dev dependencies in pyproject.toml
2. Add load_dotenv() to tests-integration/conftest.py before the env var gate

This matches the pattern already used in mcp-deepl's e2e tests and makes .env files work out of the box for integration testing.