Consider --preview flag for server subcommand

[dhruvmanila]

Summary

This PR removes the requirement of --preview flag to run the ruff server and instead considers it to be an indicator to turn on preview mode for the linter and the formatter.

resolves: #12161

Test Plan

Add test cases to assert the preview value is updated accordingly.

In an editor context, I used the local ruff executable in Neovim with the --preview flag and verified that the preview-only violations are being highlighted.

Running with:

require('lspconfig').ruff.setup({
  cmd = {
    '/Users/dhruv/work/astral/ruff/target/debug/ruff',
    'server',
    '--preview',
  },
})

The screenshot shows that E502 is highlighted with the below config in pyproject.toml:

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.