Zed collab-staging: agent_ui: Respect effort-specific thinking controls (#62480)

list_models already returns whether each effort level supports
disabling thinking. Claude Opus 5 supports disabling thinking only at
certain effort levels, so the thinking control should reflect the
capability of the selected effort level. When thinking cannot be
disabled, we keep the control visually enabled but non-toggleable and
display a tooltip explaining why and suggesting that the user select a
lower effort level.

Release Notes:

• Improved thinking controls to explain when thinking cannot be disabled
  at the selected effort level.