editor: Fix edit predictions polluting completions menu

[Dnreikronos]

Closes #49565

Before you mark this PR as ready for review, make sure that you have:

• Added a solid test coverage and/or screenshots from doing manual testing
• Done a self-review taking into account security and performance aspects
• Aligned any UI changes with the UI
  checklist

Screenshots
Before:

After:

Release Notes:

• Fixed edit predictions polluting the completions menu with unrelated snippets (e.g. Unicode symbols) when
  show_in_completions_menu is disabled (Word completions in Go files are broken despite "words": "disabled" setting #49565)

[Dnreikronos]

1. Hardcoded true in trigger_completion_on_input: The trigger_in_words parameter was ignored in the default
   match arm, always passing true to open_or_update_completions_menu. This caused word characters to
   unconditionally trigger the completions menu, even when edit predictions were hidden from it.
2. Snippet completions bypassed load_provider_completions gate: Extension snippets (e.g. Unicode symbols like
   "Theta", "tau", "turnstile") were loaded regardless of whether the trigger validation passed. This meant
   snippets polluted the completions menu even when the provider determined completions shouldn't load.

Impact

• Users with show_in_completions_menu: false will no longer see the completions menu triggered on every
  keystroke
• Extension snippets now respect the same trigger validation as other provider completions
• Users who want to remove specific snippets (e.g. Unicode symbols) can still uninstall the relevant extension