workspace: Fix panel toggle visibility checks

[mateioprea]

Objective

We're fixing toggle behavior for different panels and consolidate that into one simple workspace toggle_panel function. This PR is based on the issue raised in #63243. I saw that all of the panels behaved the same.

Solution

The solution was to consolidate a the check behind a simple workspace function: toggle_panel that check if a specific panel is visible and decides what to do next: if its visible it will simply close it. otherwise it will follow the open & focus branch.
I also added a function for checking if a panel is open.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

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Release Notes:

• N/A or Added/Fixed/Improved ...