feat: add setting to disable completion

[imtiyazakiwat]

Summary

Adds ansible.completion.enabled (default true) so users who use the
extension mainly for syntax highlighting can turn off completion entirely.
This was requested in #837, where a maintainer indicated a PR would be
welcome.

Implementation

Follows the existing ansible.validation.enabled pattern:

• ansible.completion.enabled declared in the existing Completion section
  of contributes.configuration. No new configuration section was added, since
  packages/ansible-language-server/test/globalSetup.ts reads
  contributes.configuration[6] by index.
• Typed in ExtensionSettings.completion and CompletionSettingsWithDescription,
  with the default and description in SettingsManager.
• doCompletion reads settings and returns [] at the top of the function,
  deliberately above the schema-completion branch, so schema-backed files
  such as meta/main.yml are covered too.
• doCompletionResolve returns the item untouched when the setting is off.
• The two existing ansible.completion.* toggles now cross-reference the new
  setting in their descriptions, matching how ansible.validation.lint.enabled
  refers to ansible.validation.enabled.

Note: this suppresses items offered by this extension. VS Code's own
word-based suggestions remain governed by editor.wordBasedSuggestions, and
Lightspeed inline suggestions stay under
ansible.lightspeed.suggestions.enabled.

Tests

Four cases added to completionProvider.test.ts:

1. items are offered while enabled (sanity)
2. no items once disabled
3. schema-based completions are suppressed too, asserting the stubbed schema
   completer is not consulted a second time, which pins the guard's position
   ahead of the schema branch
4. doCompletionResolve leaves insertText undefined when disabled

Verification

• vitest run over completionProvider, completionResolver and
  settingsManager suites: 77 passed, 57 skipped (@ee container tests), exit 0
• task lint: 19 hooks passed, 0 failed
• task e2e and task wdio were not run locally; leaving this as a draft for CI

fixes: #837

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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