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

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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

[TateWalker]

@maxdeviant could I get some eyes on this? Since it hasn't been merged yet, Zed auto-installs the official version every time I open it up, slowing down my development workflow.