Chat input: move + button, show attachments bar via implicitContext setting (#297118)

* Chat input: add setting for attachment bar visibility, move + icon, adjust toolbar gap

- Add `chat.attachmentBar.alwaysShow` setting (default true) to control
  whether the attachments bar is always visible above the chat input
- When enabled, shows implicit context pill in all modes (not just Ask)
- When disabled, attachments bar only appears with explicit attachments
- Move Add Context (+) button to far left of input toolbar (order -1)
- Increase gap between left toolbar items to 6px

* Consolidate: use chat.implicitContext.enabled instead of separate setting

Remove chat.attachmentBar.alwaysShow setting. Instead, the attachments
bar visibility is now driven by the existing chat.implicitContext.enabled
setting (always/first/never) combined with chat.implicitContext.suggestedContext.

When suggestedContext is true, the implicit context pill shows whenever
hasValue is true — which already respects the always/first/never semantics
of chat.implicitContext.enabled. No separate setting needed.

* Fix: preserve implicit context when no code editor is active

When chat.implicitContext.enabled is 'always' but no code editor is
active (e.g. Settings UI is focused), preserve existing implicit context
values instead of clearing them. This ensures the attachment bar stays
visible when toggling the setting back to 'always'.

* Remove custom gap override for input toolbar