Replace flake8 and isort with ruff, and introduce ruff formatter

[kitagry]

Description

Replace flake8 and isort with ruff, which handles linting, import sorting, and formatting in a single fast tool.

Previously the project had no formatter configured. This PR also introduces ruff format to establish a consistent code style.

Changes:

• Add [tool.ruff] configuration to pyproject.toml (line-length=160, E/F/W/I rules)
• Replace [testenv:flake8] and [testenv:isort] with [testenv:ruff] in tox.ini
• Remove [flake8] section from tox.ini
• Update GitHub Actions workflow to use ruff instead of flake8
• Fix Python 2 unicode builtin usage in salesforce.py (→ str)
• Apply ruff check --fix and ruff format across the codebase

Motivation and Context

flake8 and isort are maintained separately and require separate configuration. ruff replaces both with a single tool that is significantly faster and already listed in the lint dependency group. Adding a formatter also removes ambiguity around code style.

Have you tested this? If so, how?

uvx ruff check . and uvx ruff format --check . both pass with no errors.

Note for reviewers: The commits chore: apply ruff check --fix auto-fixes and chore: apply ruff format are fully automated changes with no manual edits. You can safely skip reviewing those and focus on the configuration and manual fix commits.

Optional: If you prefer single quotes (') over double quotes ("), ruff can enforce that by adding quote-style = "single" to [tool.ruff.format] in pyproject.toml. Happy to add it if there's a preference.

[dlstadther]

I'm a big fan of ruff (vs the combination of flake8/isort/black). The first PR to apply the lint and format rules to a repo that otherwise hasn't had them is hard to review, so thank you very much for separating the auto-applied commits from the manual fixes!