Fix DAP binary resolution for remote worktrees

[TateWalker]

Problem

get_dap_binary passed raw binary names ("flutter", "dart", "fvm")
as commands without resolving them against the host's PATH. WASM extensions
cannot read $PATH directly, so the debug adapter failed to launch on remote
(SSH) worktrees.

Fix

Resolve binary paths using worktree.which(), which works correctly on both
local and remote worktrees. This is consistent with how language_server_binary
already resolves the dart binary.

Related Issues

• Dart debug adapter get_dap_binary resolves locally instead of on the remote host #93

[cla-bot]

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[TateWalker]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[TateWalker]

@MrSubidubi checking to see if anything on this needs changing

[MrSubidubi]

I see the brief description above, but would you mind explaining how this prevents the auto-install and also on the $PATH-part? Does this happen because if we boot the remote server CLI, we do not load the full user path and hence need the which-API which does just that?

[TateWalker]

To clarify, I meant that when I open Zed, it auto-replaces my custom dart extension (this one) with the official one, rolling back my changes. I then have to uninstall and reinstall my dev extension to resume remote debugging work. That issue isn't related to this PR, just stating a separate concern.

On the $PATH question: yes, exactly. The remote server process may not load the full login shell environment, so $PATH can be incomplete. WASM extensions are also sandboxed and can't reliably read environment variables like $PATH directly via std::env::var. worktree.which() resolves binaries through the host process' environment context, which works correctly regardless of whether the worktree is local or remote. This is how the language server binary already handled it:

// src/dart.rs line 37
if let Some(path) = worktree.which("dart") {

[MrSubidubi]

Alright, let's go with it. Sorry for stalling this for so long, and thank you so much!