Fix preview button acting on the focused editor instead of its own pane

[austincummings]

Closes #54951

Problem

The quick action bar's preview button (markdown, SVG, and CSV behind the feature flag) resolved everything through workspace.active_item() — the globally focused item — instead of the item of the pane the button sits in. With A.md in the left pane and focus in B.md in the right pane:

• Clicking the eye button in A.md's toolbar opened a preview of B.md, in the right pane.
• Focusing a non-previewable file (e.g. B.sh) in one pane hid the button in every pane's toolbar, making A.md unpreviewable by mouse.

Clicking the button dispatched the OpenPreview action, whose workspace-level handler re-resolved the focused editor — so a click on a specific pane's toolbar was functionally identical to pressing the keybinding, discarding the pane the click happened in.

Fix

Make the flow pane-explicit end to end:

• Visibility: each pane's QuickActionBar resolves the preview type from its own active_item (set via set_active_pane_item) instead of the workspace's focused item.
• Click: no more action dispatch. The handler resolves its pane via Workspace::pane_for and calls new pane-explicit helpers — open_preview_in_pane / open_preview_to_the_side_of_pane — extracted from the action-handler bodies in all three preview crates. Keyboard actions keep their focus-based semantics and route through the same helpers with the focused editor and active pane.
• Alt-click (open in split) now splits relative to the button's pane via the new Workspace::adjacent_pane_of (the existing adjacent_pane delegates to it).

Notably not done: focusing the button's pane and re-dispatching the action. workspace.active_pane only updates when pane focus-in listeners fire at the end of the next draw, while dispatched actions run before it — the handler would still read the stale pane.

Additional changes

• Existing-preview lookup now happens before view construction, instead of building a full preview view (subscriptions, initial parse) and discarding it when one already exists.
• The focus flag now also applies to the activate-existing branch, so open-to-the-side never steals focus — previously the first invocation left focus in the editor but a repeat invocation focused the existing preview (and cancelled collaborator-following in that pane as a side effect).
• SVG handlers no longer double-check is_svg_file; CSV handlers reuse resolve_active_item_as_csv_editor instead of inlining it; is_markdown_file takes &App instead of a needless &mut Context<V>.

Testing

• New regression test preview_opens_for_the_given_pane_not_the_focused_editor reproducing the issue's setup (two panes, focus in the second, preview invoked for the first pane's editor), asserting the preview opens in the invoking pane bound to that pane's editor with the focused pane untouched.
• Markdown/SVG/CSV preview suites and the full workspace suite pass; ./script/clippy is clean.

Release Notes:

• Fixed the preview button (Markdown/SVG) previewing the focused file instead of the file in the pane the button belongs to when multiple panes are open

[ChristopherBiscardi]

thanks!