Use default settings if initialization options is empty or not provided

[dhruvmanila]

Summary

This PR fixes the bug to avoid flattening the global-only settings for the new server.

This was added in #11497, possibly to correctly de-serialize an empty value ({}). But, this lead to a bug where the configuration under the settings key was not being read for global-only variant.

By using #[serde(default)], we ensure that the settings field in the GlobalOnly variant is optional and that an empty JSON object {} is correctly deserialized into GlobalOnly with a default ClientSettings instance.

fixes: #11507

Test Plan

Update the snapshot and existing test case. Also, verify the following settings in Neovim:

1. Nothing

ruff = {
  cmd = {
    '/Users/dhruv/work/astral/ruff/target/debug/ruff',
    'server',
    '--preview',
  },
}

2. Empty dictionary

ruff = {
  cmd = {
    '/Users/dhruv/work/astral/ruff/target/debug/ruff',
    'server',
    '--preview',
  },
  init_options = vim.empty_dict(),
}

3. Empty settings

ruff = {
  cmd = {
    '/Users/dhruv/work/astral/ruff/target/debug/ruff',
    'server',
    '--preview',
  },
  init_options = {
    settings = vim.empty_dict(),
  },
}

4. With some configuration:

ruff = {
  cmd = {
    '/Users/dhruv/work/astral/ruff/target/debug/ruff',
    'server',
    '--preview',
  },
  init_options = {
    settings = {
      configuration = '/tmp/ruff-repro/pyproject.toml',
    },
  },
}

[codspeed-hq]

CodSpeed Performance Report

Merging #11566 will improve performances by 11.03%

_{Comparing dhruv/avoid-flatten (0b9fd71) with main (6be00d5)}

Summary

⚡ 1 improvements
✅ 29 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dhruv/avoid-flatten	Change
⚡	parser[numpy/ctypeslib.py]	1.2 ms	1 ms	+11.03%

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

If we have none. Should we add a test for an empty settings object?

[dhruvmanila]

There is a test :)

ruff/crates/ruff_server/src/session/settings.rs

Lines 710 to 715 in 0b9fd71

	#[test]
	fn test_empty_init_options_deserialize() {
	let options: InitializationOptions = deserialize_fixture(EMPTY_INIT_OPTIONS_FIXTURE);
	assert_eq!(options, InitializationOptions::default());
	}