Add command to pool all open tabs into a multibuffer - #1

Summary

Implements a new command PoolOpenTabsIntoMultibuffer that collects all open editor tabs and displays them together in a single multibuffer view.

Problem

Users working with multiple related files often need to view or search across all open tabs simultaneously. Previously, there was no built-in way to consolidate all open tabs into a single view.

Solution

Added a new workspace command that:

• Iterates through all open editors in the workspace
• Collects the singleton buffers (individual file buffers) from each editor
• Creates a new multibuffer containing the full content of all open files
• Opens the multibuffer in a new editor tab titled "All Open Tabs"

Usage

Users can invoke this command from the Command Palette (Cmd/Ctrl+Shift+P):

1. Open the Command Palette
2. Search for "Pool Open Tabs Into Multibuffer"
3. Execute the command

A new editor tab will appear showing all open files' contents with headers indicating each filename.

Implementation Details

The implementation follows the same pattern as the existing OpenSelectionsInMultibuffer command and reuses the open_locations_in_multibuffer infrastructure that's already used by search results and other features.

Changes:

• Added action definition in crates/editor/src/actions.rs
• Implemented handler method in crates/editor/src/editor.rs
• Registered action with workspace initialization

The implementation is minimal (34 lines), non-breaking, and follows existing code patterns in the editor.

Closes #<issue_number>

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