workspace: Fix cross-host window reuse

[hamishc]

Objective

Opening a remote folder on one host can silently activate an already-open window connected to a different host that has an identically-named path, instead of connecting.

Repro:

1. Open /foo/bar on ssh host A, then open a project on host B in the same window (host A's workspace stays retained in the background).
2. From a new window, use Open Recent to open /foo/bar on host B.
3. The existing window is activated still showing host A's project - no connection attempt, no error.

Cause: find_existing_workspace filters candidate windows by host, but then path-scores every workspace held in a qualifying window without checking each workspace's own location, so a request for host B can match host A's workspace by path alone. The window fallbacks in open_paths and the --wait branch had the same flaw - they used the window's displayed workspace unchecked, so zed could resolve a local path on a remote host.

Solution

• Match each held workspace against the requested location before path scoring in find_existing_workspace.
• Make the open_paths and --wait window fallbacks target a workspace at the requested location (preferring the displayed one) instead of the displayed one unconditionally.
• Skip non-local projects in the CLI's paths_in_existing_workspace scan.
• Log when an existing window is reused instead of connecting, to make future reports diagnosable.

Testing

• New regression test (test_find_existing_workspace_requires_matching_location) covering the scoring loop and the --wait fallback in a mixed-location window; verified it fails without the fix.
• cargo test -p workspace -p recent_projects, cargo check -p zed, and ./script/clippy all pass.
• Tested on Linux only; behavior is platform-independent (window/workspace matching logic).
• Reviewers can repro with two SSH hosts sharing a folder path using the steps above.

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

Release Notes:

• Fixed opening a remote folder activating a window connected to a different host with the same folder structure, instead of connecting.